2015MNRAS.451.2263Z

We report the discovery and characterization of a new M-dwarf binary, with component masses and radii of M_1 = 0.244_-0.003^+0.003   M_☉, R_1 = 0.261_-0.009^+0.006   R_☉, M_2 = 0.179_-0.001^+0.002   M_☉, R_2 = 0.218_-0.011^+0.007   R_☉, and orbital period of ∼ 4.1 d. The M-dwarf binary HATS551-027 (LP 837-20) was identified as an eclipsing binary by the HATSouth survey, and characterized by a series of high-precision photometric observations of the eclipse events, and spectroscopic determinations of the atmospheric parameters and radial velocity orbits. HATS551-027 is one of few systems with both stellar components lying in the fully convective regime of very low mass stars, and can serve as a test for stellar interior models. The radius of HATS551-027A is consistent with models to 1σ, whilst HATS551-027B is inflated by 9 percent at 2σ significance. We measure the effective temperatures for the two stellar components to be Teff, 1 = 3190±100 K and Teff, 2 = 2990±110 K; both are slightly cooler than theoretical models predict, but consistent with other M-dwarfs of similar masses that have previously been studied. We also measure significant Hα emission from both components of the binary system, and discuss this in the context of the correlation between stellar activity and the discrepancies between the observed and model temperatures.