The triple degenerate star WD 1704+481

WD 1704+481 is a visual binary in which both components are white dwarfs. W e present spectra of the H alpha line of both stars which show that one com ponent (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 st ar. From radial velocity measurements of the close binary we find an orbita l period of 0.1448 d, a mass ratio, qM(bright)M(faint), of 0.70 +/- 0.03 an d a difference in the gravitational redshifts of 11.5 +/- 2.3 km s(-1). The masses of the close pair of white dwarfs predicted by the mass ratio and g ravitational redshift difference combined with theoretical cooling curves a re 0.39 +/- 0.05 and 0.56 +/- 0.07 M-.. WD 1704+481 is therefore also likel y 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.