The Evolution of Cataclysmic Variables as Revealed by their Donor Stars
  We reconstruct the evolutionary path followed by cataclysmic variables (CVs)
from the observed mass-radius relationship of their donor stars. Along the way,
we update the semi-empirical CV donor sequence of Knigge (2006) and present a
comprehensive review of the link between CV evolution and donor physics. After
calibrating state-of-the art stellar models for use in the CV setting, we fit
self-consistent theoretical evolution sequences to the observed donor masses
and radii. In the standard model of CV evolution, AML below the period gap is
assumed to be driven solely by gravitational radiation (GR), while AML above
the gap is usually described by a magnetic braking prescription due to
Rappaport, Verbunt & Joss (1983). We find that simple scaled versions of these
recipes match the data quite well. However, the optimal scaling factors turn
out to be f_GR = 2.47 +/- 0.22 below the gap and f_MB = 0.66 +/- 0.05 above.
The implications and applications of our results include: (1) The revised
evolution sequence yields correct locations for the CV minimum period and the
upper edge of the period gap; the standard sequence does not. (2) A comparison
of predicted and observed WD temperatures suggests an even higher value for
f_GR, but this is sensitive to the assumed WD mass. (3) The absolute donor
magnitudes predicted by our sequences can be used to set firm lower limits on
the distances toward CVs. (4) Both standard and revised sequences predict that
short-period CVs should be susceptible to dwarf nova (DN) eruptions, consistent
with observations. However, both sequences also predict that the DNe fraction
among long-period CVs should decline with P_orb. Observations suggest the
opposite behaviour. (5) The ratio of long-period CVs to short-period,
pre-bounce CV is about 3x higher for the revised sequence than the standard
one. This may resolve a long-standing problem in CV evolution. [abridged]
