Stellar models of evolved secondaries in cataclysmic variables

In this paper we study the impact of chemically evolved secondaries on cataclysmic variable ( CV) evolution. We find that when evolved secondaries are included, a spread in the secondary mass-orbital period plane comparable to that seen in the data is produced for either the saturated prescription for magnetic braking or the unsaturated model commonly used for CVs. We argue that in order to explain this spread, a considerable fraction of all CVs should have evolved stars as secondaries. The evolved stars become fully convective at lower orbital periods. Therefore, even if there were an abrupt decrease in magnetic braking for fully convective stars ( contrary to open cluster data), it would not be expected to produce a sharp break in the period distribution for CVs. We also explore recent proposed revisions to the angular momentum loss rate for single stars and find that only modest increases over the saturated prescription are consistent with the overall observed spin-down pattern. We compare predictions of our models with diagnostics of the mass accretion rate in white dwarfs and find results intermediate between the saturated and the older braking prescription. Taken together, these suggest that the angular momentum loss rate may be higher in CV secondaries than in single stars of the same rotation period but still significantly lower than in the traditional model. Alternative explanations for the CV period gap are discussed.