Spectroscopic Evidence for Starspots on the Secondary Star of SS Cygni

Abstract

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