The double degenerate system NLTT 11748

We show that the extremely low-mass white dwarf NLTT 11748 (0.17 M-circle dot) is in a close binary with a fainter companion. We obtained a series of radial velocity measurements of the low-mass white dwarf using the Ha core and determined an orbital period of 5.64 h. The velocity semi-amplitude (K = 274.8 kms(-1)) and orbital period imply that it is a degenerate star, and that the minimum mass for the companion is 0.75 M-circle dot (assuming a mass of 0.167 M-circle dot for the primary). Our analysis of Balmer line profiles shows that a 0.75 M-circle dot white dwarf companion does not contribute more than 2% or 5% of the flux (V-band) for helium-or hydrogen-rich surfaces, respectively. The kinematics of the system suggest that it belongs to the Galactic halo.