2.4 RS CVn stars

RS CVn stars represent a class of close detached binaries with the more massive primary component being a G-K giant or subgiant and the secondary a subgiant or dwarf of spectral classes G to M. They show optical variability (outside eclipses) which is characterised by an amplitude up to $0.6 mag$ in the V band and interpreted as the rotationally modulated effect of cool spots on their surfaces. Because of low luminosity of the secondary many RS CVn systems appear as single-line binaries which is attractive for spectral analysis. In binaries, where both components can be observed, the primary appears more active than the secondary.

The identification of a new class of variable stars, named after the prototype system RS CVn, was proposed by Hall (1976 ) who identified the following classification signatures:

photometric variability;
$Ca II H & K$ emission lines;
subgiant component well within its Roche lobe;
fast rotation, i.e. almost synchronised binaries with orbital periods of a few days;
orbital period variations.

Since they are tidally locked close binaries, they are also fast rotators. Thus, similar to other cool active stars, RS CVn-type variables are remarkable due to strong chromospheric plages, coronal X-ray, and microwave emissions, as well as strong flares in the optical, UV, radio, and X-ray.

Large amplitude brightness variations of RS CVn stars imply the presence of enormous starspots on their surfaces covering up to 50% of the visible disc (see Section 5). Remarkable activity and high luminosity of these stars make them favourite targets for light curve modelling, Doppler imaging and spectral line analysis. Most of the present knowledge on starspots is based on studies of this type stars.