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 × 10−7) d (2 h 10 min). The UB light curves display an ellipsoidal modulation with amplitudes of ≈ 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. Rcomp ≈ 0.01 R⊙) and therefore degenerate. We modelled the UB light curves and derived i = 78.05° ± 0.50° and a mass ratio of q = Mcomp/MsdB = 0.87 ± 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 MsdB = 0.72 ± 0.26 M⊙ and Mcomp = 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 Teff = 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.