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 cataclysmic variables ( CVs ) , including those which form with a brown dwarf donor .
Our models confirm the mismatch between the calculated minimum period ( P _ { min } \simeq 70 min ) and the observed short–period cut–off ( \simeq 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 .