RS Canum Venaticorum Binaries

Abstract

Some properties of the RS CVn suntype of eclipsing binaries can be explained in terms of observational selection. Provided that the components of a binary are (a) the sameage, and (b) too small relative to their Roche lobes to have begun to undergo any mass transfer, then aclipses deep enough to be detected are most likely to be found in systems where (i) one star is a subgiant of spectral type late G to early K (ii) the other star is near the main sequence but above it, and (iii) the binary period is a few days. These conditions will only be satisfied if the more evo lved component is about 3 percent more massive than the companion, in accordance with observations. Other features of RS CVn systems, such as emission lines, and secular changes in the light curve, are then to be interpreted as consequences of, but not causes of, the evolutionary status of these objects