Spectroscopic studies of the small-amplitude Cepheid SU Cas

A new set of 16 high-resolution spectra for the small-amplitude Cepheid SU Cas obtained in 2007–2009 has allowed us to determine its atmospheric parameters (T _eff = 6345 ± 30 K, log g = 2.40, V _t = 3.25 km s^?1) and to measure its radial velocities. The latter were added to the general list of radial velocities (375 estimates) obtained in the last 90 years. Using a frequency analysis, we have refined the pulsation and orbital periods of the Cepheid. Apart from the well-known fundamental pulsation period , we have detected a possible secondary period of . Their ratio of 0.96 suggests the existence of nonradial pulsations in the Cepheid’s atmosphere. Based on photoelectric photometry in the last 60 years, we have shown that the effective temperature undergoes cyclic secular changes of ±200 K with an unknown period. The mean effective temperature T _eff = 6395 ± 52 K estimated from photometric data agrees well with our estimate from spectroscopic data. The variations of the mean color index, effective temperature, and γ-velocity (in 90 years of observations) point to a possible orbital motion of the well-known hot companion with the most probable periods of , , and . The elemental abundances in the atmosphere of SU Cas confirm the conclusion that this Cepheid is a typical yellow supergiant after the first dredge-up. Our T _eff estimate gives a radius of 32R _⊙ and a distance of 455 pc for it, which is inconsistent with its membership in the open cluster Alessi 95. The question about the pulsation mode of SU Cas still remains open.