Secondary stars in CVs - the observational picture

Recent theoretical and observational progress has substantially improved the definition of the lower main sequence and established a new basis for a comparison of main sequence stars and the secondaries in CVs. The evolutionary sequences of Kolb & Baraffe [1999] imply that the secondaries in many CVs are expanded compared with main sequence stars of the same mass as a consequence of unusually high mass transfer rates and/or pre-CV nuclear evolution. We show that the location of the secondaries of all well-studied CVs in the spectral type period diagram implies that they are consistent with having near-solar metallicities. We show, furthermore, that the surface brightness of K/M stars depends on gravity and metallicity and present new Barnes-Evans relations valid for dwarfs of near-solar metallicity and the secondaries in CVs of the galactic disk population. Distances derived by the surface brightness method agree with recent measurements of the trigonometric parallaxes of a few selected systems.

K. Beuermann · 1999