  Tidal dissipation in compact white dwarf (WD) binary systems significantly
influences the physical conditions (such as surface temperature and rotation
rate) of the WDs prior to mass transfer or merger. In these systems, the
dominant tidal effects involve the excitation of gravity waves and their
dissipation in the outer envelope of the star. We calculate the amplitude of
tidally excited gravity waves in low-mass (0.3M_\odot) helium-core (He) WDs as
a function of the tidal forcing frequency \omega. Like carbon-oxygen (CO) WDs
studied in our previous paper, we find that the dimensionless tidal torque
F(\omega) (inversely proportional to the effective tidal quality factor) has an
erratic dependence on \omega. On average, F(\omega) scales approximately as
\omega^6, and is several orders of magnitude smaller for He WDs than for CO
WDs. We find that tidal torques can begin to synchronize the WD rotation when
the orbital period is less than about a hour, although a nearly constant
asynchronization is maintained even at small periods. We examine where the
tidally excited gravity waves experience non-linear breaking or resonant
absorption at a critical layer, allowing us to estimate the location and
magnitude of tidal heating in the WD envelope. We then incorporate tidal
heating in the MESA stellar evolution code, calculating the physical conditions
of the WD as a function of orbital period for different WD models. We find that
tidal heating makes a significant contribution to the WD luminosity for
short-period (~10 min) systems such as SDSS J0651+2844. We also find that for
WDs containing a hydrogen envelope, tidal heating can trigger runaway hydrogen
shell burning, leading to a nova-like event before the onset of mass transfer.
