THE ACTIVE RS-CANUM-VENATICORUM BINARY II-PEGASI - I - STELLAR AND ORBITAL PARAMETERS

A detailed model atmosphere analysis of high-resolution and high S/N C CD spectra of II Peg has yielded for the first time a self-consistent set of fundamental parameters of the primary component: T-eff=4600 K, log g=3.2, [M/H]=-0.4, xi(t)=2.0 km s(-1). In addition, 121 new high q uality radial velocity measurements(1) allowed us to determine improve d orbital parameters, resulting in a new orbital ephemeris of T-conj=2 449582.9268+6.724333E. The position of the primary of II Peg in the HR diagram with the new parameters corresponds to a K2 IV star with mass approximate to 0.8 M.. The evolved character of the star is confirmed by the C/N ratio, which is reduced significantly relative to the sola r value. The unspotted V magnitude of the star of 6.(m)9 is estimated from the observed variations of the TiO bands and quasi-simultaneous p hotometry. The blend of Li I 6707 Angstrom is suspected to vary in equ ivalent width due to spot modulation, The lithium abundance for the un spotted star, Li/H=1.0+/-0.1, is found to be consistent with other pos t-main sequence chromospherically active stars. Combining all paramete rs, the radius R approximate to 3.4 R. and the inclination i approxima te to 60 degrees of the primary are estimated with the assumption that its rotational axis is perpendicular to the orbital plane. The second ary is probably a M0-M3 V star with a mass of about 0.4 M..