  We present follow-up spectroscopy of 21 cataclysmic variables (CVs) with
evolved secondaries and ongoing or recently-terminated mass transfer.
Evolutionary models predict that the secondaries should have anomalous surface
abundances owing to nuclear burning in their cores during their main-sequence
evolution and subsequent envelope stripping by their companion white dwarfs. To
test these models, we measure sodium (Na) abundances of the donors from the
Fraunhofer "D" doublet. Accounting for interstellar absorption, we find that
{\it all} objects in our sample have enhanced Na abundances. We measure 0.3
$\lesssim$ [Na/H] $\lesssim$ 1.5 dex across the sample, with a median [Na/H] =
0.956 dex, i.e., about an order of magnitude enhancement over solar values. To
interpret these values, we run MESA binary evolution models of CVs in which
mass transfer begins just as the donor leaves the main sequence. These
generically predict Na enhancement in donors with initial donor masses $\gtrsim
1\,M_{\odot}$, consistent with our observations. In the models, Na enrichment
occurs in the donors' cores via the NeNa cycle near the end of their
main-sequence evolution. Na-enhanced material is exposed when the binaries
reach orbital periods of a few hours. Donors with higher initial masses are
predicted to have higher Na abundances at fixed orbital period owing to their
higher core temperatures during main-sequence evolution. The observed [Na/H]
values are on average $\approx$0.3 dex higher than predicted by the models.
Surface abundances of evolved CV donors provide a unique opportunity to study
nuclear burning products in the cores of intermediate-mass stars.
