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 M_ʘ). 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 M_ʘ. 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 M_ʘ. 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.