The Secondary Stars of Cataclysmic Variables
  I review what we know about the donor stars in cataclysmic variables (CVs),
focusing particularly on the close link between these binary components and the
overall secular evolution of CVs. I begin with a brief overview of the
"standard model" of CV evolution and explain why the key observables this model
is designed to explain - the period gap and the period minimum -- are
intimately connected to the properties of the secondary stars in these systems.
CV donors are expected to be slightly inflated relative to isolated, equal-mass
main-sequence (MS) stars, and this "donor bloating" has now been confirmed
observationally. The empirical donor mass-radius relationship also shows a
discontinuity at M_2 = 0.2 M_sun which neatly separates long- and short-period
CVs. This is strong confirmation of the basic disrupted magnetic braking
scenario for CV evolution. The empirical M_2-R_2 relation can be combined with
stellar models to construct a complete, semi-empirical donor sequence for CVs.
This sequence provides all physical and photometric properties of "normal" CV
secondaries along the standard CV evolution track. The observed donor
properties can also be used to reconstruct the complete evolution track
followed by CVs, i.e. the mass-transfer rate and angular-momentum-loss rate as
a function of orbital period. Such a reconstruction suggests that angular
momentum loss rates below the period gap are too high to be driven solely by
gravitational radiation.
