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 \pm 0.000 000 000 9 days , or about three hours .
Analysis of our time-series spectroscopy reveals a velocity semi-amplitude of K _ { 1 } = 71.6 \pm 1.7 km s ^ { -1 } for the sdB and best-fitting atmospheric parameters of T _ { eff } = 28900 \pm 500 K , log g = 5.64 \pm 0.06 , and log N ( He ) / N ( H ) = -2.5 \pm 0.2 .
Although we can not claim a unique solution from modeling the light curve , the best–fitting model has an sdB mass of 0.45 M _ { \sun } and a cool companion mass of 0.12 M _ { \sun } .
These results are roughly consistent with a canonical–mass sdB and M dwarf separated by a \sim 0.84 R _ { \sun } .
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 | \dot { P } | > 7.2 \times 10 ^ { -12 } .
If EC 10246-2707 evolves into a cataclysmic variable , its period should fall below the famous CV period gap .