KPD 0422+5421: A new short-period subdwarf B white dwarf binary

The subdwarf B (sdB) star KPD 0422+5421 was discovered to be a single-lined spectroscopic binary with a period of P = 0.090 1795 +/- (3 x 10(-7)) d (2 h 10 min). The U and B light curves display an ellipsoidal modulation with amplitudes of approximate to 0.02 mag. The sdB star contributes nearly all of the observed flux. This and the absence of any reflection effect suggest that the unseen companion star is small (i.e. R-comp approximate to 0.01 R-.)and therefore degenerate. We modelled the U and B light curves and derived i = 78.05 degrees +/- 0.50 degrees and a mass ratio of q = M-comp/M-sdB = 087 +/- 0.15. The sdB star fills 69 per cent of its Roche lobe. These quantities may be combined with the mass function of the companion [f(M)=0.126 +/- 0.028 M-.] to derive M-sdB = 0.72 +/- 0.26 M-comp = 0.62 +/- 0.18 M-.. We used model spectra to derive the effective temperature, surface gravity and helium abundance of the sdB star. We found T-eff = 25 000 +/- 1500 K, log g = 5.4 +/- 0.1 and [He/H] = -1.0. With a period of 2 h 10 min, KPD 0422+5421 has one of the shortest known orbital periods of a detached binary. This system is also one of only a few known binaries that contain a subdwarf B star and a white dwarf. Thus KPD 0422+5421 represents a relatively unobserved, and short-lived, stage of binary star evolution.