The long-period RS Canum Venaticorum binary IM Pegasi - I. Orbital and stellar parameters

New high-resolution and high signal-to-noise ratio spectroscopic observatio ns carried out in 1996-1997 are analysed. A total of 85 new high quality ra dial velocity measurements are used for determining the new orbital paramet ers: T-conj=HJD2450342.883+24.64880E, e=0.0, gamma=-14.09 km s(-1), K-1=34. 39 km s(-1). A model atmosphere analysis of the averaged spectrum of the st ar has yielded a self-consistent set of fundamental parameters of the prima ry component: T-eff=4450 K, log g=2.4, [M/H]=0.0, xi(t)=1.6 km s(-1), upsil on sini=26.5 km s(-1). The primary is found to be a typical K2 III giant wi th the mass of about 1.5 M. which has undergone the first convective mixing on the Red Giant Branch ([C/H]=-0.32, [N/H]=0.30, C/N=1.15). The unspotted V magnitude of the star of 5.(m)55 is estimated from the observed variatio ns of the TiO band and quasi-simultaneous photometry. Combining all paramet ers, the radius and inclination of the primary as well as a probable spectr al class of the secondary are estimated.