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Published online by Cambridge University Press: 14 August 2015
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 0d.21 (dwarf Cepheids). Belong to the population of the disk, are absent in clusters. Their luminosity is 2m–3m 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, 0m.1 (rarely up to 0m.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 0d.2. A typical representative – δSct.