The triple degenerate star WD 1704+481

WD 1704+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 (WD 1704+481.2=Sanduleak B=GR 577) is a close binary with two white dwarf components. Thus, WD 1704+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, qM_brightM_faint, of 0.70+/-0.03 and a difference in the gravitational redshifts of 11.5+/-2.3kms^-1. 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+/-0.05 and 0.56+/-0.07M_solar. WD 1704+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.