  We use an independent new sample of cataclysmic variables (CVs), constructed
by selecting objects for Halpha emission, to constrain the properties of the
intrinsic CV population. This sample is restricted to systems that are likely
to be non-magnetic and unevolved; it consists of 17 CVs, of which at least 10
have orbital periods above 3 h. We find that even very generous allowance for
selection effects is not sufficient to reconcile the large ratio of short- to
long-period CVs predicted by standard CV evolution theory with the observed
sample, possibly implying that short-period systems evolve faster than
predicted by the disrupted magnetic braking model. This would require that an
angular momentum loss mechanism, besides gravitational radiation, acts on CVs
with orbital periods below the period gap. To bring the model into agreement
with observations, the rate of angular momentum loss below the period gap must
be increased by a factor of at least 3, unless the model also over estimates
the angular momentum loss rate of long-period CVs.
