Spectroscopic Binary Cepheid AU Peg in the Gaia DR3 Epoch: Effect of Ellipsoidality, Mass, and Evolutionary Status

The physical characteristics and evolutionary status of the spectroscopic binary Cepheid AU Peg, usually classified as a BL Her variable, is revised. Taking into account previously unpublished radial velocity measurements made with a radial velocity meter (RVM), the parameters of the relative orbit of the Cepheid and its companion are refined. Based on the available radial velocity measurements and photometric observations, a number of estimates of the Cepheid’s pulsating radius are made. Based on 74 new radial velocity measurements made in the period JD2453930–2459490 virtually simultaneously with the Gaia DR3 photometric observations (44 brightness measurements in the period JD256962–2457877), the mean radius of AU Peg is determined by the pulsating photosphere method equal to (16.0 ± 0.6) \({{R}_{ \odot }}\); the amplitude of radius variations is about 0.7 \({{R}_{ \odot }}\). For the first time, 2% brightness variations are found for a spectroscopic binary Cepheid in the G band with a half orbital period, the phases of which are clearly synchronized with the phases of the orbital motion (conjunctions and quadratures) and can be interpreted as a manifestation of the ellipsoidality effect of the main component filling the Roche sphere at 60–70%. The orbit of the binary system calculated on the basis of astrometric data from Gaia DR3 and radial velocities is limited by a torus with a thickness of |z| ≤ 300 pc and inner and outer radii equal to 7.2 and 8.8 kpc, respectively, while the vertical velocity lies within |V_z| ≤ 18 km/s. Based on the kinematics and abundance of heavy elements, AU Peg can certainly be classified as a representative of the typical population of a thin disk of moderate (∼1–3 Gyr) age, which completely excludes its classification as a BL Her variable. The Cepheid, which current mass, as follows from the magnitude of ellipsoidality, does not exceed ∼(0.85 ± 0.05) \({{M}_{ \odot }}\), is at the evolutionary stage after the mass exchange in the subgiant phase with a companion - a main-sequence star, which is currently significantly more massive than the Cepheid.