The Chemical Composition of S-Cepheids and Double-Mode Cepheids

The main aim of this work is to consider the evolutionary status of s-Cepheids (SC), - small-amplitude Cepheids with smooth sinusoidal light, colour and radial velocity curves. The unusual character of these objects could be explained in two ways: (1) SCs are crossing the Cepheid instability strip for the first time and they are first (or high) overtone pulsators (Efremov 1970); (2) SCs have close early-type companions, which affect gravitationally the Cepheid He II zone. Such an effect can decrease the Cepheid pulsational activity (Lloyd Evans, 1968). According to theoretical calculations, low helium abundance in Cepheid atmospheres, which are crossing the instability strip for the first time, could produce small-amplitude oscillations. On the other hand, double-mode Cepheids (DMC) show oscillations with two (or three) periods: fundamental with large-amplitude asymmetrical light, colour and radial velocity curves, and first overtone with smooth sinusoidal small-amplitude ones. Nevertheless, detailed spectral and photometric investigations permit the detection of companions for the brightest SCs, and support the second hypothesis.