Spectroscopic Evidence for Starspots on the Secondary Star of SS Cygni

Abstract

Cataclysmic variables are interacting binary systems where a cool,rapidly rotating secondary star passes material to a white dwarf. Ifthis mass loss is to continue, then there must be continuous angularmomentum loss from the system. By analogy with the Sun and other coolstars, it has been assumed that magnetic braking is responsible, angularmomentum being carried away by an ionized wind from the secondary star,threading a dynamo-generated magnetic field. We have discovered TiOabsorption in the spectrum of SS Cyg, whose secondary star should be toohot to show such features. The most likely explanation of its presenceis cool star spots caused by the strong (0.1 T) fields required by themagnetic braking theories.