  We have discovered a detached pair of white dwarfs (WDs) with a 12.75 min
orbital period and a 1,315 km/s radial velocity amplitude. We measure the full
orbital parameters of the system using its light curve, which shows ellipsoidal
variations, Doppler boosting, and primary and secondary eclipses. The primary
is a 0.25 Msun tidally distorted helium WD, only the second tidally distorted
WD known. The unseen secondary is a 0.55 Msun carbon-oxygen WD. The two WDs
will come into contact in 0.9 Myr due to loss of energy and angular momentum
via gravitational wave radiation. Upon contact the systems may merge yielding a
rapidly spinning massive WD, form a stable interacting binary, or possibly
explode as an underluminous supernova type Ia. The system currently has a
gravitational wave strain of 10^-22, about 10,000 times larger than the
Hulse-Taylor pulsar; this system would be detected by the proposed LISA
gravitational wave mission in the first week of operation. This system's rapid
change in orbital period will provide a fundamental test of general relativity.
