  We explore the observational appearance of the merger of a low-mass star with
a white dwarf (WD) binary companion. We are motivated by Schreiber et al.
(2016), who found that multiple tensions between the observed properties of
cataclysmic variables (CVs) and standard evolution models are resolved if a
large fraction of CV binaries merge as a result of unstable mass transfer.
Tidal disruption of the secondary forms a geometrically thick disk around the
WD, which subsequently accretes at highly super-Eddington rates. Analytic
estimates and numerical hydrodynamical simulations reveal that outflows from
the accretion flow unbind a large fraction >~ 90% of the secondary at
velocities ~500-1000 km/s within days of the merger. Hydrogen recombination in
the expanding ejecta powers optical transient emission lasting about a month
with a luminosity > 1e38 erg/s, similar to slow classical novae and luminous
red novae from ordinary stellar mergers. Over longer timescales the mass
accreted by the WD undergoes hydrogen shell burning, inflating the remnant into
a giant of luminosity ~300-5000 L_sun, effective temperature T_eff ~ 3000 K and
lifetime ~1e4-1e5 yr. We predict that ~1e3-1e4 Milky Way giants are CV merger
products, potentially distinguishable by atypical surface abundances. We
explore whether any Galactic historical slow classical novae are masquerading
CV mergers by identifying four such post-nova systems with potential giant
counterparts for which a CV merger origin cannot be ruled out. We address
whether the historical transient CK Vul and its gaseous/dusty nebula resulted
from a CV merger.
