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.
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.