Low-mass white dwarfs need friends: five 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 Mʘ). The high success rate of our observations supports the notion that evolution within a binary star is needed to obtain white dwarfs with masses below 0.45 Mʘ. We have measured the orbital period and radial velocity amplitude of four of our discoveries. No late-type features are seen in any of our targets at a level which forces the mass of any main-sequence companion to be less than 0.1 Mʘ. This, together with our measured mass functions, implies that the companion stars are in fact white dwarfs. Our observations raise the number of detached, double-degenerate close binaries with known orbital periods from two to six. All of the orbits are circular, 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 4h, but with the 1 cycle d–1 alias at 4.8 h also a possibility. Gravitational radiation will cause this star to merge within about 2 ×109 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 ejection of the envelope during the common-envelope phase is very efficient. The spectrum of the companion is directly detectable in 1713+332 but in none of the other systems.