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 CCD spectra of II Peg has yielded for the first time a self-consistent set of fundamental pa- rameters of the primary component: Te =4600 K, log g=3.2, (M/H)=-0.4, t=2.0 km s 1 . In addition, 121 new high qual- ity radial velocity measurements 1 allowed us to determine im- proved orbital parameters, resulting in a new orbital ephemeris of Tconj=2449582.9268+6.724333E. The position of the pri- mary of II Peg in the HR diagram with the new parameters cor- responds to a K2 IV star with mass 0.8 M. The evolved character of the star is confirmed by the C/N ratio, which is reduced significantly relative to the solar 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 photome- try. The blend of Lii 6707 ˚ A is suspected to vary in equivalent width due to spot modulation. The lithium abundance for the unspotted star, Li/H=1.00.1, is found to be consistent with other post-main sequence chromospherically active stars. Com- bining all parameters, the radius R 3.4 R and the inclination i 60 of the primary are estimated with the assumption that its rotational axis is perpendicular to the orbital plane. The sec- ondary is probably a M0-M3 V star with a mass of about 0.4 M.