  The population synthesis of cataclysmic variables below the period is
investigated. A grid of detailed binary evolutionary sequences has been
calculated and included in the simulations to take account of additional
angular momentum losses beyond that associated with gravitational radiation and
mass loss, due to nova outbursts, from the system. As a specific example, we
consider the effect of a circumbinary disk to gain insight into the ingredients
necessary to reproduce the observed orbital period distribution. The resulting
distributions show that the period minimum lies at about 80 minutes with the
number of systems monotonically increasing with increasing orbital period to a
maximum near 90 minutes. There is no evidence for an accumulation of systems at
the period minimum which is a common feature of simulations in which only
gravitational radiation losses are considered. The period distribution is found
to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The
steepness of the lower edge of the period gap can be reproduced, for example,
by an input of systems at periods near 2.25 hrs due to a flow of cataclysmic
variable binary systems from orbital periods longer than 2.75 hrs. The good
agreement with the cumulated distribution function of observed systems within
the framework of our model indicates that the angular momentum loss by a
circumbinary disk or a mechanism which mimics its features coupled with a
weighting factor to account for selection effects in the discovery of such
systems and a flow of systems from above the period gap to below the period gap
are important ingredients for understanding the overall period distribution of
cataclysmic variable binary systems.
