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The Discovery of a Short Period Double-Degenerate Binary Star

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.