  We report the discovery and characterisation of a new M-dwarf binary, with
component masses and radii of M1 = 0.244 -0.003/+0.003 Msun, R1 = 0.261
-0.009/+0.006 Rsun, M2 = 0.179 -0.001/+0.002 Msun, R2 = 0.218 -0.011/+0.007
Rsun, and orbital period of ~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 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 Halpha 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.
