LOW-MASS WHITE-DWARFS NEED FRIENDS - 5 NEW DOUBLE-DEGENERATE CLOSE BINARY STARS

We have discovered five detached close binary stars out of seven white dwarfs chosen for their low mass (< 0.45 M.). The high success rate o f our observations supports the notion that evolution within a binary star is needed to obtain white dwarfs with masses below 0.45 M.. We ha ve measured the orbital period and radial velocity amplitude of four o f our discoveries. No late-type features are seen in any of our target s at a level which forces the mass of any main-sequence companion to b e less than 0.1 M.. This, together with our measured mass functions, i mplies that the companion stars are in fact white dwarfs. Our observat ions raise the number of detached, double-degenerate close binaries wi th known orbital periods from two to six. All of the orbits are circul ar, a consequence of past interaction. We find periods of 1.1, 3.3 and 4.8 d for 1713+332, 1241-010 and 1317+453. 2331+290 has a very short period, most likely 4 h, but with the 1 cycle d(-1) alias at 4.8 h als o a possibility. Gravitational radiation will cause this star to merge within about 2 x 10(9) yr. Close double-degenerates go through one or more stages during which the two stars orbit inside a common envelope . The long orbital periods of 1241-010 and 1317+453 suggest that the e jection of the envelope during the common-envelope phase is very effic ient. The spectrum of the companion is directly detectable in 1713+332 but in none of the other systems.