  We have found that the white dwarf PG 1101+364 is a double-lined white
dwarf/white dwarf binary with an orbital period of 3.47 hours. PG 1101+364 is
the shortest period detached double-degenerate yet found and gravitational
radiation will cause it to merge in 2.5 X 10**9 years. PG 1101+364 has a mass
ratio of 0.87 +/- 0.03. From the relative strengths of the narrow core of
Halpha, the lighter and therefore larger star appears to contribute more of the
light by an amount that indicates almost equal temperatures for the two stars.
We chose to observe PG 1101+364 on the basis of its low spectroscopic mass
(0.31 Msun), and the discovery of its binary nature confirms the belief that
binary evolution is required to produce such low mass white dwarfs. The radial
velocity semi-amplitudes (69.7 +/- 1.7 km/s and 80.3 +/- 1.6 km/s) and the
spectroscopic mass show that we see the orbit of PG 1101+364 at a low
inclination of ~ 25 deg.
