The triple degenerate star WD 1704 + 481

WD1704+481 is a visual binary in which both components are white dwarfs. We present spectra of the Hα line of both stars which show that one component (WD1704+481.2 = Sanduleak B = GR577) is a close binary with two white dwarf components. Thus, WD1704+481 is the first known triple degenerate star. From radial velocity measurements of the close binary we find an orbital period of 0.1448d, a mass ratio, q = Mbright Mfaint of q = 0.70±0.03 and a difference in the gravitational redshifts of 11.5±2.3km s. The masses of the close pair of white dwarfs predicted by the mass ratio and gravitational redshift difference combined with theoretical cooling curves are 0.39±0.05M⊙ and 0.56±0.07M⊙ . WD1704+481 is therefore also likely to be the first example of a double degenerate in which the less massive white dwarf is composed of helium and the other white dwarf is composed of carbon and oxygen.