• gravitational waves;
  • binaries: close;
  • stars: individual: SDSS J163030.58+423305.8;
  • supernovae: general;
  • white dwarfs


We report the discovery of a new detached, double white dwarf (WD) system with an orbital period of 39.8 min. We targeted SDSS J163030.58+423305.8 (hereafter J1630) as part of our radial velocity programme to search for companions around low-mass WDs using the 6.5-m MMT. We detect peak-to-peak radial velocity variations of 576 km s−1. The mass function and optical photometry rule out main-sequence companions. In addition, no millisecond pulsar companions are detected in radio observations. Thus the invisible companion is most likely another WD. Unlike the other 39-min binary SDSS J010657.39−100003.3, follow-up high-speed photometric observations of J1630 obtained at the McDonald 2.1-m telescope do not show significant ellipsoidal variations, indicating a higher primary mass and smaller radius. The absence of eclipses constrain the inclination angle to i≤ 82°. J1630 contains a pair of WDs, 0.3 M primary +≥0.3 M invisible secondary, at a separation of ≥0.32 R. The two WDs will merge in less than 31 Myr. Depending on the core composition of the companion, the merger will form either a single core He-burning subdwarf star or a rapidly rotating massive WD. The gravitational wave strain from J1630 is detectable by instruments like the Laser Interferometer Space Antenna (LISA) within the first year of operation.