Evidence for reduced magnetic braking in polars from binary population
  models
  We present the first population synthesis of synchronous magnetic cataclysmic
variables, called polars, taking into account the effect of the white dwarf
(WD) magnetic field on angular momentum loss. We implemented the reduced
magnetic braking (MB) model proposed by Li, Wu & Wickramasinghe into the Binary
Stellar Evolution (BSE) code recently calibrated for cataclysmic variable (CV)
evolution. We then compared separately our predictions for polars and
non-magnetic CVs with a large and homogeneous sample of observed CVs from the
Sloan Digital Sky Survey. We found that the predicted orbital period
distributions and space densities agree with the observations if period
bouncers are excluded. For polars, we also find agreement between predicted and
observed mass transfer rates, while the mass transfer rates of non-magnetic CVs
with periods ${\gtrsim3}$ hr drastically disagree with those derived from
observations. Our results provide strong evidence that the reduced MB model for
the evolution of highly magnetized accreting WDs can explain the observed
properties of polars. The remaining main issues in our understanding of CV
evolution are the origin of the large number of highly magnetic WDs, the large
scatter of the observed mass transfer rates for non-magnetic systems with
periods ${\gtrsim3}$ hr, and the absence of period bouncers in observed
samples.
