Brown dwarfs and the cataclysmic variable period minimum

Using improved, up-to-date stellar input physics tested against observation s of low-mass stars and brown dwarfs, we calculate the secular evolution of low-mass donor cataclysmic variables (CVs), including those that form with a brown-dwarf donor. Our models confirm the mismatch between the calculate d minimum period (P-min similar or equal to 70 min) and the observed short- period cut-off(similar or equal to 80min) in the CV period histogram. We fi nd that tidal and rotational corrections applied to the one-dimensional ste llar structure equations have no significant effect on the period minimum. Theoretical period distributions synthesized from our model sequences alway s show an accumulation of systems at the minimum period, a feature absent f rom the observed distribution. We suggest that non-magnetic CVs become unob servable as they are effectively trapped in permanent quiescence before the y reach P-min and that small-number statistics may hide the period spike fo r magnetic CVs.