EC 10246-2707: a new eclipsing sdB + M dwarf binary⋆

Abstract

We announce the discovery of a new eclipsing hot subdwarf B + M dwarf binary, EC 10246-2707, and present multi-colour photometric and spectroscopic observations of this system. Similar to other HW Vir-type binaries, the light curve shows both primary and secondary eclipses, along with a strong reflection effect from the M dwarf; no intrinsic light contribution is detected from the cool companion. The orbital period is 0.118 507 993 6 ± 0.000 000 000 9 days, or about three hours. Analysis of our time- series spectroscopy reveals a velocity semi-amplitude of K1 = 71.6 ± 1.7 km s−1 for the sdB and best-fitting atmospheric parameters of Teff = 28900 ± 500 K, log g = 5.64 ± 0.06, and log N(He)/N(H) = -2.5 ± 0.2. Although we cannot claim a unique solution from modeling the light curve, the best–fitting model has an sdB mass of 0.45 M⊙ and a cool companion mass of 0.12 M⊙. These results are roughly consistent with a canonical–mass sdB and M dwarf separated by a ∼ 0.84 R⊙. We find no evidence of pulsations in the light curve and limit the amplitude of rapid photometric oscillations to < 0.08%. Using 15 years of eclipse timings, we construct an O-C diagram but find no statistically significant period changes; we rule out | ˙P | > 7.2×10−12. If EC 10246- 2707 evolves into a cataclysmic variable, its period should fall below the famous CV period gap.