Low-mass stellar and substellar companions to sdB stars

Abstract

It has been suggested that besides stellar companions, substellar objects in
close orbits may be able to trigger mass loss in a common envelope phase and
form hot subdwarfs. In an ongoing project we search for close substellar
companions combining time resolved high resolution spectroscopy with
photometry. We determine the fraction of as yet undetected radial velocity
variable systems from a sample of 27 apparently single sdB stars to be 16%. We
discovered low-mass stellar companions to the He-sdB CPD-20 1123 and the
pulsator KPD 0629-0016. The brown dwarf reported to orbit the eclipsing binary
SDSS J0820+0008 could be confirmed by an analysis of high resolution spectra
taken with UVES. Reflection effects have been detected in the light curves of
the known sdB binaries CPD-64 481 and BPS CS 22169-0001. The inclinations of
these systems must be much higher than expected and the most likely companion
masses are in the substellar regime. Finally, we determined the orbit of the
sdB binary PHL 457, which has a very small radial velocity amplitude and may
host the lowest mass substellar companion known. The implications of these new
results for the open question of sdB formation are discussed.

Full text

Page 1

arXiv:1112.2929v1 [astro-ph.SR] 13 Dec 2011
**Volume Title**
ASP Conference Series, Vol. **Volume Number**
**Author**
c ?**Copyright Year** Astronomical Society of the Pacific
Low-mass stellar and substellar companions to sdB stars
S. Geier1, L. Classen1, P. Br¨ unner1, K. Nagel1, V. Schaffenroth1, C. Heuser1,
U. Heber1, H. Drechsel1, H. Edelmann1, C. Koen2, S. J. O’Toole3,
L. Morales-Rueda4
1Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute,
Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7,
D-96049 Bamberg, Germany
2Department of Statistics, University of the Western Cape, Private Bag X17,
Bellville 7535, South Africa
3Australian Astronomical Observatory, PO Box 296, Epping, NSW, 1710,
Australia
4Department of Astrophysics, Faculty of Science, Radboud University
Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, NE
Abstract.
in close orbits may be able to trigger mass loss in a common envelope phase and form
hot subdwarfs. In an ongoing project we search for close substellar companions com-
bining time resolved high resolution spectroscopy with photometry. We determine the
fractionofas yetundetectedradial velocityvariablesystems froma sample of27 appar-
ently single sdB stars to be ≃ 16%. We discovered low-mass stellar companions to the
He-sdB CPD−20◦1123 and the pulsator KPD0629−0016. The brown dwarf reported
to orbit the eclipsing binary SDSSJ0820+0008 could be confirmed by an analysis of
high resolution spectra taken with UVES. Reflection effects have been detected in the
light curves of the known sdB binaries CPD−64◦481 and BPSCS22169−0001. The
inclinations of these systems must be much higher than expected and the most likely
companion masses are in the substellar regime. Finally, we determined the orbit of the
sdB binary PHL457, which has a verysmall radial velocityamplitude and may host the
lowest mass substellar companionknown. The implications of these new results for the
open question of sdB formation are discussed.
It has been suggested that besides stellar companions, substellar objects
1. Introduction
About half of the sdB stars reside in close binaries with periods ranging from a few
hours to a few days (Maxted et al. 2001; Napiwotzki et al. 2004). Because the compo-
nents’ separation in these systems is much less than the size of the subdwarf progenitor
during its red-giant phase, these systems must have experienced a common-envelope
and spiral-in phase (Han et al. 2002, 2003).
Although the common-envelope ejection channel is not yet properly understood
in detail, it provides a reasonable explanation for the strong mass loss required to form
sdB stars. However, for about half of all known subdwarfs there is no evidence for close
stellar companions as no radial velocity variations are found. Among other formation
1

Page 2

2
S. Geier et al.
scenarios, the merger of two helium white dwarfs has often been suggested to explain
the origin of single sdB stars (Han et al. 2002, 2003).
Soker (1998) suggested that substellar objects like brown dwarfs and planets may
also be swallowed by their host star and that common envelope ejection could form
hot subdwarfs. Substellar objects with masses higher than ≃ 10 MJwere predicted to
survive the common envelope phase and end up in a close orbit around the stellar rem-
nant, while planets with lower masses would entirely evaporate or merge withthe stellar
core. The stellar remnant is predicted to lose most of its envelope and evolve towards
the extreme horizontal branch (EHB). A similar scenario has been proposed to explain
the formation of apparently single low mass white dwarfs (Nelemans & Tauris 1998).
The discovery of a brown dwarf with a mass of 0.053 ± 0.006 M⊙in an 0.08d orbit
around such a white dwarf supports this scenario and shows that substellar companions
can influence the outcome of stellar evolution (Maxted et al. 2006).
The planet discovered to orbit the sdB pulsator V391Peg with a period of 1170d
and a separation of 1.7AU was the first planet found to have survived the red-giant
phase of its host star (Silvotti et al. 2007). Serendipitous discoveries of two substellar
companions around the eclipsing sdB binary HWVir (Lee et al. 2009) and one brown
dwarf around the similar system HS0705+6700 (Qian et al. 2009) followed. These
substellar companions to hot subdwarfs have rather wide orbits, were not engulfed by
the red giant progenitors and therefore could not have influenced the evolution of their
host stars. But the fact that substellar companions in wide orbits around sdBs seem to
be common suggests that similar objects closer to their host stars might exist as well
(for a review see Schuh 2010).
Herewepresent new results from our ongoing search for sdBswithclose substellar
companions using high-resolution, time-resolved spectroscopy as well as time-resolved
photometry. First we determine the fraction of as yet undetected RV variable systems
from a sample of apparently single sdB stars and review the known candidate systems.
Then we present newly discovered systems with low-mass stellar as well as substellar
companions and discuss the implications of these systems for sdB formation.
2. Are all ’single’ sdBs RV variable?
The most important drawback of the radial velocity (RV) method is the unknown incli-
nation of the binaries. From an RV curve of a single-lined binary alone only a lower
limit for the companion mass can be derived. For a single object it is therefore im-
possible to prove the existence of a substellar companion, because a more massive
stellar companion seen at low inclination cannot be excluded. Most sdB binaries are
single-lined systems and only lower limits can be put on the masses of their compan-
ions usually assuming a canonical EHB mass of ≃ 0.47 M⊙. Since about 50% of all
known sdBs are in close binary systems with stellar companions, there must be a cer-
tain number of such systems seen at low inclinations. Furthermore, most RV variable
systems have been identified from medium resolution time-resolved spectroscopy (e.g.
Copperwheat et al. 2011) and the fraction of binaries with variabilities too small to be
detected at such resolutions is therefore not well constrained. In principle, all appar-
ently single sdBs could have close, but yet undetected companions.
A large sample of sdBs has to be studied to decide whether the fraction of systems
with small RV variations is consistent with the low-inclination extension of the known
sdB binary population or not. A higher fraction than expected would be an indication

Page 3

Low-mass stellar and substellar companions to sdB stars
3
for a population of substellar companions. Up to now our sample consists of 27 bright
single-lined sdB stars. We used high resolution spectra (R = 48000) obtained with
ESO-2.2m/FEROS. Each star has been observed several times and the timespans be-
tween the observations range from days to years. We chose a set of sharp, unblended
metal lines with accurate rest wavelengths, fitted Gaussian and Lorentzian profiles to
the metal lines and determined the RVs. The errors ranged from ≃ 0.3 to 2.0kms−1. To
check the wavelength calibration for systematic errors we used telluric features as well
as nightsky emission lines.
Four stars were found to show significant RV variability. The RV shifts range
from 0.5 to 12.7kms−1. Follow-up photometry and high resolution spectroscopy are
necessary to exclude pulsational variability and derive the orbital parameters of these
binaries. Constraints can then be put on the companion masses. No significant RV
variations were found in the rest of the sample. We deduce that any undetected RV
variation of a programme star has to be lower than its RV measurement uncertainty.
Soker (1998) suggests that these objects should be more massive than 10 MJ, otherwise
they would have been destroyed during the CE phase, and should have orbital periods
of the order of ≃ 10d.
Adopting this period we derive an upper limit for the binary mass function, which
translates into an upper limit for M2sini if we assume the canonical mass of 0.47 M⊙
for the sdB. In this way tight constraints can be put on possible substellar companions.
Substellar companions with M2sini = 5−35 MJcan be excluded. In conclusion ≃ 16%
of the apparently single stars in our sample show RV variations. The true fraction is
expected to be higher, because the accuracy is limited by the S/N of the spectra in most
cases. Substellar companions in close orbits can be most likely excluded in ≃ 84% of
our sample.
Our RV survey showed that most of the apparently single sdB stars do indeed
not show RV variations of more than ≃ 1.0kms−1on timescales from days to years.
Nevertheless, ≃ 16% do show small RV variations. However, it has still to be shown
whether these variations are due to orbital motion.
3. Candidate sdB systems with substellar companions
3.1. Candidates from literature
Several close binary systems with possible substellar companions have been found. A
companion in the planetary mass range was reported to orbit the bright sdB HD149382
based on an RV curve with very small amplitude (P = 2.39d,K = 2.3kms−1). How-
ever, Jacobs et al. (2011) and Norris et al. (2011) did not detect significant RV vari-
ability in this period range and excluded the presence of a planetary companion. We
obtained high resolution follow-up spectra with AAT/CYCLOPS (R = 80000). Con-
sistent with the results of Jacobs et al. (2011) and Norris et al. (2011) we were not able
to verify the RV variability on timescales of a few days reported in Geier et al. (2009).
The eclipsing binary AADor (P ≃ 0.26d, K = 40kms−1) has a companion
with a mass close to the hydrogen burning limit (Rauch et al. these proceedings).
PG1329+159 is a reflection effect binary with very similar orbital parameters (P ≃
0.25d, K = 40kms−1) and a minimum companion mass (0.08 M⊙) right at the border
region between stars and brown dwarfs (Morales-Rueda et al. 2003; Geier et al. 2010).

Page 4

4
S. Geier et al.
The very short-period reflection effect binary PG1017−086 (P ≃ 0.073d, K =
51kms−1)mayhaveabrowndwarf companion (> 0.05 M⊙,Maxted et al. 2002;Geier et al.
2010) as well. The very similar but eclipsing system J1622+4730 (P ≃ 0.07d, K =
47kms−1) has been found in the course of the MUCHFUSS project (Geier et al. these
proceedings).
The reflection effect binary KBS13 (KIC1868650) most likely has a substellar
companion as well. For et al. (2008) determined not only the orbital parameters of
this binary (P ≃ 0.29d, K = 23kms−1), but also the projected rotational velocity and
surface gravity of the sdB (vrotsini = 23.3 ± 1.1kms−1, logg = 5.87). Assuming the
canonical mass for the sdB as well as orbital synchronisation of the sdB, the mass of
the companion can be calculated as described in Geier et al. (2010) to be 0.055 M⊙.
Because the rotational velocity of sdBs in binaries with such low-mass companions has
been found to be slower than predicted for a synchronised orbit (see Sect. 3.3, 3.4, 3.5),
the companion mass derived for KBS13 must be regarded as an upper limit, while a
lower limit is provided by the binary mass function (0.046 M⊙). The companion mass
of KBS13 is therefore tightly constrained. The extremely accurate Kepler light curve
of this star can be used to verify this conclusion (Østensen et al. 2010).
3.2.Low-mass stellar companions to CPD−20◦1123 and KPD0629−0016
CPD−20◦1123 (Albus1)wasdiscovered tobeabright He-sdB by Vennes et al. (2007).
Except for the unique double-lined spectroscopic binary PG1544+488 (Ahmad et al.
2004) none of the known He-sdBs is known to reside in a close binary system. We
obtained medium resolution time resolved spectroscopy (R = 3400) of this star using
ESO-NTT/EMMI and found it to be RV variable. Follow-up spectroscopy was taken
with FEROS and an orbital solution could be determined (P = 2.3d, K = 44.3kms−1).
The minimum mass of the unseen companion is 0.21 M⊙. Whether the companion is a
compact object like a white dwarf or a low-mass MS star is unclear. Due to the rather
long period a reflection effect indicative of a cool MS star would not be detectable.
CPD−20◦1123 is the first He-sdB with a close unseen companion.
KPD0629−0016 is the only known pulsating sdB star (type sdBVs) in the CoRoT
field (Charpinet et al. 2010, and references therein). We detected RV variability of
the order of K ≃ 60kms−1based on medium resolution spectra taken with ESO-
NTT/EMMI and ESO-NTT/EFOSC2, which are too high to be caused by pulsations.
The orbital period could not yet be determined from available data, but is most likely
shorter than one day, and the minimum mass of the unseen companion is of the order
of 0.2 M⊙. More observations are needed to obtain a unique orbital solution and clarify
the nature of the companion. The high-precision CoRoT light curve may be of great
help in this respect.
3.3.SDSSJ0820+0008 - Brown dwarf confirmed
The eclipsing sdB binary SDSSJ0820+0008 was reported to host a brown dwarf com-
panion (Geier et al. 2011d). In order to verify these results and to constrain the binary
parameters better we obtained 3.5hr of consecutive time resolved high resolution spec-
troscopy with ESO-VLT/UVES (R ≃ 40000) covering the whole orbit of the system.
No spectral features from the irradiated companion could be detected. The orbital pa-
rameters derived from the new dataset (P = 0.097d, K = 47.0 ± 1.2kms−1see Fig. 1)
are in perfect agreement with the previous results from Geier et al. (2011d). Atmo-
spheric parameters of the sdB primary are determined for the first time with high ac-

Page 5

Low-mass stellar and substellar companions to sdB stars
5
Figure 1.
from high resolution spectra taken with UVES.
Phased RV curve of SDSSJ0820+0008. The RVs have been measured
curacy (Teff = 25900K, logg = 5.42) from spectra taken during the total eclipse of
the companion. Comparing the surface gravity derived from the spectroscopic analysis
with the value derived from the light curve solution the mass of both binary compo-
nents can be constrained. With the new results we can exclude an sdB mass of more
than ≃ 0.5 M⊙and a companion mass exceeding ≃ 0.07 M⊙confirming the substellar
nature of the companion.
Using the UVES spectra we were able to measure the projected rotational velocity
of the sdB for the first time with high accuracy (vrotsini = 67 ± 2kms−1). In contrast
to our expectations and despite the very short period of the system (Geier et al. 2010)
this velocity turned out to be much too low for the sdB to rotate synchronously with
its orbital motion (vrotsinisyncro≃ 120kms−1). Another interesting result is a signif-
icant shift of the system velocity ∆γ = +15 ± 1kms−1with respect to the results of
Geier et al. (2011d). A systematic zero-point shift of this order is rather unlikely. An-
other explanation would be the presence of a third unseen component in the system.
More observations are needed to investigate this further.
3.4.CPD−64◦481 - Reflection effect reveals brown dwarf companion
Edelmann et al. (2005) discovered CPD−64◦481 to be a close sdB binary with very
small RV amplitude and concluded that the companion may be a substellar object. We
measured the RVs of CPD−64◦481 from FEROS spectra as described in Sect. 2 and
determined the orbital parameters (P = 0.277263±0.000005d, K = 23.90±0.05kms−1
see Fig. 2 upper panel), which are perfectly consistent with the parameters derived by
Edelmann et al. (2005) using the same data. Geier et al. (2010) measured a very low
vrotsini of the subdwarf primary and derived a much higher companion mass in the

Page 6

6
S. Geier et al.
stellar regime assuming tidally locked rotation. In this scenario, the binary would be
seen nearly pole-on (i ≃ 7◦).
However, photometric follow-up revealed that this scenario must be incorrect. We
took a multi-colour light curve with the the SAAO STE4 CCD camera mounted on
the SAAO 1.0-m telescope and detected a sinusoidal variation with the orbital period
characteristic for a reflection effect (see Fig. 2 lower panel). This modulation is caused
by the changing light contribution of the irradiated companion when orbiting around
the hot primary. Such a reflection effect is only detectable if the binary has a high
inclination, which means that the low inclination determined in Geier et al. (2010) must
be highly underestimated, and that the sdB rotates much slower than synchronisation.
The inclination of the binary will be constrained by a light curve analysis (e.g.
Geier et al. 2011d). Preliminary solutions favour inclinations between 60◦and 70◦.
Adopting the canonical sdB mass of 0.47 M⊙the mass of the unseen companion most
likely ranges between 0.050 M⊙and 0.055 M⊙, well below the stellar mass limit. The
detection of a reflection effect in the light curve of CPD−64◦481 therefore revealed
that the companion of CPD−64◦481 is most likely a brown dwarf and that the rotation
of the sdB cannot be tidally locked.
3.5.BPSCS22169−0001 - Small reflection effect and non-synchronised rotation
BPSCS22169−0001 was discovered to be a close sdB binary with very small RV am-
plitude by Edelmann et al. (2005). The conclusion that the companion may be substel-
lar was again questioned by Geier et al. (2010), who measured a low vrotsini, assumed
tidal synchronisation of the sdB and constrained the companion mass to be stellar.
Again, a very small reflection effect (0.1%) was detected in a light curve of this binary
obtained atSAAO,which allowed ustoconstrain the orbital period better (P ≃ 0.214d).
The new period is somewhat longer than the one given in Edelmann et al. (2005) which
was based on just a few FEROS spectra. Adopting this period and using the nine
FEROS spectra as well as four spectra obtained with the Coude spectrograph mounted
at the 2.7m McDonald telescope we derive an RV-semiamplitude K = 16.2kms−1.
Dropping the assumption of orbital synchronisation the minimum mass of the un-
seen companion is as low as 0.026 M⊙, but due to the very small amplitude of the
light curve variation the inclination angle is expected to be smaller than in the case of
CPD−64◦481. Anyway, the inclination would have to be smaller than ≃ 25◦to lift the
companion above the stellar mass limit. A combined spectroscopic and photometric
analysis is necessary to constrain the companion mass more tightly.
3.6. PHL457 - The lowest mass substellar companion?
PHL457 was discovered to be RV variable by Edelmann et al. (2005), but the data
were not sufficient to determine the orbital parameters. We obtained 26 additional
spectra with FEROS and derived the orbital solution with high statistical significance
(P = 0.3128 ± 0.0007d, K = 12.80 ± 0.08kms−1see Fig. 3). The minimum mass of
the unseen companion is 0.026 M⊙and therefore very well below the stellar mass limit.
Furthermore, Blanchette et al. (2008) found PHL457 to be a long period sdB pulsator
(sdBVs) with a possible reflection effect present in the light curve. The latter is impor-
tant because it hints at a quite high inclination, which means that the actual companion
mass cannot be much higher than the lower limit. However, the data are not sufficient
to clarify this issue (Green priv. comm.).

Page 7

Low-mass stellar and substellar companions to sdB stars
7
0.96
0.965
0.97
0.975
0.98
0.985
0.99
0.995
1
-0.3-0.2-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
normalized light
orbital phase
Figure 2.
panel: Light curves of CPD−64◦481 taken at SAAO (B,R,V-band) phased to the or-
bital period. The reflection effect is clearly visible as well as the absence of eclipses.
Upper panel: Phased Radial velocity curve of CPD−64◦481. Lower

Page 8

8
S. Geier et al.
Figure 3. Phased Radial velocity curve of PHL457.
4. Conclusion
In Fig. 4 the orbital parameters of all known reflection effect sdB binaries are plotted.
From the 21 systems known, 11 may have substellar companions. Since only about
100 close sdB binaries with orbital solutions are known, the fraction of systems with
low-mass companions exceeds 20%. The fraction of possible substellar companions is
higher than 10%. Due to selection effects, the true fraction must be higher. Reflection
effects or eclipses necessary to clarify the nature of the companions are more likely to
be found in close and highly inclined binaries. Recent discoveries of low-mass objects
around sdBs with periods of more than ten days (Geier et al. 2011c; Barlow et al. 2011)
indicate that such systems do exist as well and that they might be quite numerous.
It can be seen in Fig. 4 that the minimum companion mass seems to decrease with
longer orbital periods. This is contrary to what is expected from common envelope
evolution theory: The smaller the mass of the companion, the more the system has to
shrink before enough energy and angular momentum is transferred to the common en-
velope to be ejected. However, this scenario assumes that the companion survives the
CE phase. But the fate of substellar objects engulfed by close red giants is unclear.
Soker (1998) argues that substellar objects might also evaporate during the CE phase
or merge with the core of the red giant (see also Politano et al. 2008). Most recently,
we discovered the rapidly rotating single sdB EC22081−1916, the possible outcome of
such a common envelope merger event (Geier et al. 2011a, Geier et al. these proceed-
ings). The destruction of the companions during or shortly after the CE phase might
explain the lack of lowest-mass objects with very short periods seen in Fig. 4. Those
objects might have come too close to the red-giant core to survive.

Page 9

Low-mass stellar and substellar companions to sdB stars
9
Figure 4.
ted against their orbital periods. Eclipsing systems are plotted as diamonds, non-
eclipsing systems as circles. Open symbols mark all known systems from the liter-
ature (see Geier et al. 2011b, and references therein), while the filled symbols mark
the newly discovered systems presented here and in Geier et al. (these proceedings).
The solid line marks the border between the stellar and the substellar mass regime,
the dashed line the border between brown dwarfs and planets.
The RV semiamplitudes of all knownreflection effect sdB binaries plot-
We conclude that close substellar companions to sdB stars do exist and that they
are by no means rare. They play a significant role in the formation of sdB stars. They
may either form close binaries and survive, merge with the red-giant core creating
rapidly rotating single sdBs or evaporate in the red-giant envelope. In the latter case
they create ordinary single sdBs that rotate slowly (see also Geier et al. these proceed-
ings).
References
Barlow, B. N., Dunlap, B. H., & Clemens, J. C. 2011, ApJ, 737, L2
Ahmad, A., Jeffery, C. S., & Fullerton, A. W. 2004, A&A, 418, 275
Blanchette, J.-P., Chayer, P., Wesemael, F., et al. 2008, ApJ, 678, 1329
Charpinet, S., Green, E. M., Baglin, A., et al. 2010, A&A, 516, L6
Copperwheat, C. M., Morales-Rueda, L., Marsh, T. R., Maxted, P. F. L., & Heber, U. 2011,
MNRAS, 415, 1381
Edelmann, H., Heber, U., Altmann, M., Karl, C., & Lisker, T. 2005, A&A 442, 1023
For, B.-Q., Edelmann, H., Green, E. M., et al. 2008, ASP Conf. Ser., 392, 203
Geier, S., Edelmann, H., Heber, U., & Morales-Rueda, L. 2009, ApJ, 702, L25
Geier, S., Heber, U., Podsiadlowski, Ph., et al. 2010, A&A, 519, 25
Geier, S., Classen, L., & Heber, U. 2011a, ApJ, 733, L13
Geier, S., Hirsch, H., Tillich, A., et al. 2011b, A&A, 530, 28
Geier, S., Napiwotzki, R., Heber, U., & Nelemans, G. 2011c, A&A, 528, L16
Geier, S., Schaffenroth, V., Drechsel, H., et al. 2011d, ApJ, 731, L22
Han Z., Podsiadlowski P., Maxted P. F. L., Marsh T. R., & Ivanova N. 2002, MNRAS, 336, 449

Page 10

10
S. Geier et al.
Han, Z., Podsiadlowski, P., Maxted, P. F. L., & Marsh, T. R. 2003, MNRAS, 341, 669
Jacobs, V. A., Østensen, R. H., van Winckel, H., et al. 2011, AIP Conf. Ser., 1331, 304
Lee, J. W., Kim, S.-L., Kim, C.-H., et al. 2009, AJ, 137, 3181
Maxted, P. F. L., Heber, U., Marsh, T. R., & North, R. C. 2001, MNRAS, 326, 139
Maxted, P. F. L., Marsh, T. R., Heber, U., et al. 2002, MNRAS, 333, 231
Maxted, P. F. L., Napiwotzki, R., Dobbie, P. D., & Burleigh, M. R. 2006, Nature, 442, 543
Morales-Rueda, L., Maxted, P. F. L., Marsh, T. R., North, R. C., & Heber, U. 2003, MNRAS,
338, 752
Napiwotzki, R., Karl, C., Lisker, T., et al. 2004, Ap&SS, 291, 321
Nelemans, G., & Tauris, T. M. 1998, A&A, 335, L85
Norris, J. M., Wright, J. T., Wade, R. A., Mahadevan, S., & Gettel, S. 2011, ApJ, in press
(arXiv:1110.1384)
Østensen, R. H., P´ apics, P. I., Oreiro, R., et al. 2011, ApJ, 731, L13
Østensen, R. H., Silvotti, R., Charpinet, S., et al. 2010, MNRAS, 409, 1470
Politano, M., Taam, R. E., van der Sluys, M., & Willems, B. 2008, ApJ, 687, L99
Qian, S.-B., Zhu, L.-Y., Zola, S., et al. 2009, ApJ, 695, L163
Schuh, S. 2010, AN, 331, 489
Silvotti, R., Schuh, S., Janulis, R., et al. 2007, Nature, 449, 189
Soker, N. 1998, AJ, 116, 1308
Vennes, S., Kawka, A., & Smith, J. A. 2007, ApJ, 668, L59