  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 CVs, including those which form with a brown dwarf donor star.
Our models confirm the mismatch between the calculated minimum period (P_min ~
70min) and the observed short-period cut-off (~80min) in the CV period
histogram. Theoretical period distributions synthesized from our model
sequences always show an accumulation of systems at the minimum period, a
feature absent in 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. We calculate the minimum period for high mass transfer
rate sequences and discuss the relevance of these for explaining the location
of CV secondaries in the orbital period - spectral type diagram. We also show
that a recently suggested revised mass-radius relation for low-mass
main-sequence stars cannot explain the CV period gap.
