  Compact binary white dwarfs (WDs) undergoing orbital decay due to
gravitational radiation can experience significant tidal heating prior to
merger. In these WDs, the dominant tidal effect involves the excitation of
outgoing gravity waves in the inner stellar envelope and the dissipation of
these waves in the outer envelope. As the binary orbit decays, the WDs are
synchronized from outside in (with the envelope synchronized first, followed by
the core). We examine the deposition of tidal heat in the envelope of a
Carbon-Oxygen WD and study how such tidal heating affects the structure and
evolution of the WD. We show that significant tidal heating can occur in the
star's degenerate hydrogen layer. This layer heats up faster than it cools,
triggering runaway nuclear fusion. Such "tidal novae" may occur in all WD
binaries containing a CO WD, at orbital periods between 5 min and 20 min, and
precede the final merger by 10^5-10^6 years.
