Stellar activity and the period gap in cataclysmic variables

The evolution of a cataclysmic binary is computed under the assumption of angular momentum loss due primarily to a magnetic wind from the secondary star, in order to investigate the possible relation between the period gap in cataclysmic variables and the sudden decrease in magnetic activity observed in main sequence stars at B-V values of about 1.65. A decrease in the angular momentum loss rate at the point where the secondary becomes fully convective can temporarily switch off mass transfer. A period gap of the correct width and location is obtained if the decrease is rapid and large enough, and in addition if the initial relative angular momentum loss rate lies between about 4 and 7 x 10 to the -9th/year. The calculation is based on homologous stellar models calibrated with the Grossman et al. (1974) main sequence. Attention is given to the influence of star spots, by way of their effect on the radius of the secondary star.