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 0.45M(circle dot) and a cool companion mass of 0.12M(circle dot). These results are roughly consistent with a canonical-mass sdB and M dwarf separated by a similar to 0.84 R-circle dot. We find no evidence of pulsations in the light curve and limit the amplitude of rapid photometric oscillations to <0.08 per cent. Using 15 yr of eclipse timings, we construct an observed minus calculated (O - C) diagram but find no statistically significant period changes; we rule out vertical bar(P)over dot vertical bar > 7.2 x 10(-12). If EC 10246-2707 evolves into a cataclysmic variable, its period should fall below the famous cataclysmic variable period gap.