We report the discovery and characterisation of a new M-dwarf binary , with component masses and radii of M _ { 1 } = 0.244 _ { -0.003 } ^ { +0.003 } M _ { \odot } , R _ { 1 } = 0.261 _ { -0.009 } ^ { +0.006 } R _ { \odot } , M _ { 2 } = 0.179 _ { -0.001 } ^ { +0.002 } M _ { \odot } , R _ { 2 } = 0.218 _ { -0.011 } ^ { +0.007 } R _ { \odot } , and orbital period of \sim 4.1 days .
The M-dwarf binary HATS551-027 ( LP 837-20 ) was identified as an eclipsing binary by the HATSouth survey , and characterised 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 1 \sigma , whilst HATS551-027B is inflated by 9 % at 2 \sigma significance .
We measure the effective temperatures for the two stellar components to be T _ { \text { eff, 1 } } = 3190 \pm 100 K and T _ { \text { eff, 2 } } = 2990 \pm 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 H \alpha 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 .