KPD 0422 + 5421 : A New Short Period Subdwarf B / White Dwarf Binary

The sdB star KPD 0422+5421 was discovered to be a single-lined spectroscopic binary with a period of P = 0.0901795± (3×10) days (2 hours, 10 minutes). The U and B light curves display an ellipsoidal modulation with amplitudes of ≈ 0.02 magnitudes. 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 ≈ 0.01R⊙) and therefore degenerate. We modeled the U and B 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% of its Roche lobe. These quantities may be combined with the mass function of the companion (f(M) = 0.126±0.028M⊙) to derive MsdB = 0.72±0.26M⊙ and Mcomp = 0.62 ± 0.18M⊙. 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 hours and 10 minutes, 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 which 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.