THE BINARY-SYSTEM V373 CAS - ORBITAL ELEMENTS, PARAMETERS OF THE COMPONENTS, AND HELIUM ABUNDANCE

CCD spectra with high resolution and large signal-to-noise ratio are u sed to study the binary system V373 Gas, which includes two early B st ars, and is apparently undergoing a very early phase of mass exchange between the components. The new radial velocity measurements made it p ossible to improve the system's orbital elements and to refine the mas s estimates for the components: M-orb = 18.6 +/- 2.4 M. for the primar y (star A) and 14.2 +/- 1.9M. for the secondary (star B). A self-consi stent method for estimation of the effective temperature and gravitati onal acceleration yields T-eff = 23200 +/- 600 K and log g = 3.0 +/- 0 .2 for component A and T-eff = 26800 +/- 1500 K and log g = 3.5 +/- 0. 2 for component B. These parameters correspond to the spectral types B 0.5-B1 (II-Ib) for A and B0.5 III for B. The helium abundance is enhan ced in the atmospheres of both components: He/H = 0.23 +/- 0.03 and 0. 18 +/- 0.02 forA and B, respectively. This helium abundance excess con firms that He/H depends on age, and is consistent with the fact that b oth the components of V373 Cas are near the end of the main sequence s tage. The magnesium abundance for the primary is close to the solar va lue, suggesting that its metallicity may be normal. The rotation veloc ities vsini = 130 +/- 10 km/s (A) and 60 +/- 5 km/s (B) are derived fr om the He I lines. These confirm the conclusion of Hill and Fisher tha t the rotation of the components in their orbital motion is asynchrono us. The masses M-ev estimated using evolutionary tracks are overestima ted by 34% for the both components, compared to the more accurate valu es M-orb determined from the orbit analysis. This is the largest known discrepancy between M-ev and M-orb for a binary system of this type. The age of the system is 7-8 million years.