* Astronomy

Title: EC 10246-2707: a new eclipsing sdB + M dwarf binary
Authors: B. N. Barlow, D. Kilkenny, H. Drechsel, B. H. Dunlap, D. O'Donoghue, S. Geier, R. G. O'Steen, J. C. Clemens, A. P. LaCluyze, D. E. Reichart, J. B. Haislip, M. C. Nysewander, K. M. Ivarsen

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 0.1185079936 ±0.0000000009 days, or about three hours. Analysis of our time-series spectroscopy reveals a velocity semi-amplitude of K_1 = 71.6 ±1.7 km/s 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 modelling the light curve, the best-fitting model has an sdB mass of 0.45 solar masses and a cool companion mass of 0.12 solar masses. These results are roughly consistent with a canonical-mass sdB and M dwarf separated by a ~ 0.84 solar radii. 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-dot| > 7.2 x 10^(-12). If EC 10246-2707 evolves into a cataclysmic variable, its period should fall below the famous CV period gap.

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