Physical Classification of Short-Period Variables

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It is widely accepted that pulsating variables with periods less than 0.2 day fall into two distinct groups. These defined by Kukarkin et al. (1969) as, firstly, RRs:RR Lyrae type variables with the period not exceeding 0 d . 21 (dwarf Cepheids). Belong to the population of the disk, are absent in clusters. Their luminosity is 2 m –3 m fainter than the luminosity of RRab and RRc stars. A typical representative – SX Phe. and secondly, δSct:δScuti type stars. Pulsating variables of spectral class A (late sub-classes) and F, the amplitudes of light variation do not exceed, as a rule, 0 m .1 (rarely up to 0 m .3). The form of light curve strongly varies usually. According to many characteristics resemble dwarf Cepheids, but differ from them by the small amplitudes. Are met in the Hyades-type clusters. Similar to the RRs type stars their periods do not exceed 0 d .2. A typical representative – δSct.

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An examination of the nine small-amplitude, ultrashort-period variables assigned to the Hyades group, including one newly discovered (HR 3588), shows that they are divided into two luminosity groups with no apparent period-luminosity relation. A search for similar stars in the old disk population revealed one (HD 6870) new variable in the σ Puppis group. It is concluded from the space motions and, in a few cases, membership in stellar groups that ultrashort-period variables of all amplitudes occur in both the young and old disk populations and there is no strong argument that they, as well as the other blue stragglers in the old disk population, are not stars of 1.5 to 2.5 ⵙ, in the shell hydrogen-burning stage of their evolution.