Brown Dwarfs and the Cataclysmic Variable Period Minimum 3

Using improved, up–to–date stellar input physics tested against observations of low– mass stars and brown dwarfs we calculate the secular evolution of low–mass donor cat-aclysmic variables (CVs), including those which form with a brown dwarf donor. Our models confirm the mismatch between the calculated minimum period (P min ≃ 70 min) and the observed short–period cut–off (≃ 80 min) in the CV period histogram. We find that tidal and rotational corrections applied to the one–dimensional stellar structure equations have no significant effect on the period minimum. Theoretical period distributions synthesized from our model sequences always show an accumulation of systems at the minimum period, a feature absent from the observed distribution. We suggest that non–magnetic CVs become unobservable as they are effectively trapped in permanent quiescence before they reach P min , and that small–number statistics may hide the period spike for magnetic CVs.