EC 10246−2707: an eclipsing subdwarf B + M dwarf binary

We announce the discovery of a new eclipsing hot subdwarf B + M dwarf binary, EC 10246−2707, and present multicolour 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 9936 ± 0.000 000 0009 d, or about 3 h. Analysis of our time series spectroscopy reveals a velocity semi-amplitude of K 1 = 71.6 ± 1.7 km s−1 for the sdB and best-fitting atmospheric parameters of T eff = 28 900 ± 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 modelling 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 . If EC 10246−2707 evolves into a cataclysmic variable, its period should fall below the famous cataclysmic variable period gap.