The evolutionary status of RS CVn binaries

Using arguments based on stellar evolution calculations, we show that of all binaries with periods in the range 1–30 day, those most likely to show recognizable eclipses must fall mainly into three distinct categories. (1) The more massive component is $$\lesssim\,2\,{M}_{\odot}$$, has nearly but not quite evolved to fill its Roche lobe, and is at the base of the giant branch, with spectral type close to K0 IV; the companion has evolved to near or just beyond central hydrogen exhaustion, and has spectral type F–G IV–V; the mass ratio (hotter/cooler) is close to unity (~ 0.95); the period is in the range 2–10 day. (2) The originally more massive component has already evolved to fill its Roche lobe, and has transferred a large amount of mass to its companion; the mass ratio is far from unity. (3) Both components are of early spectral type (O to – B5); neither has yet evolved to fill its Roche lobe, nor evolved far from the main sequence; the mass ratio is fairly close to unity; the period is ≲5 day. The first category provides a natural explanation of several properties of the RS CVn class of Algol-like eclipsing binaries. Other properties, such as emission lines seen outside eclipse, variable distortion of the light curve, and radio and X-ray emission must, we conclude, be fairly general properties of systems which contain a late-G–K–M star which is close to, but does not fill, its Roche lobe.