NGTS J214358.5-380102 -- NGTS discovery of the most eccentric known M-Dwarf binary system

We present the discovery of NGTS J214358.5-380102, an eccentric M-dwarf binary discovered by the Next-Generation Transit Survey (NGTS). The system period of 7.618 d is greater than many known eclipsing M-dwarf binary systems. Its orbital eccentricity of 0.323(-0.0037)(+0.0014) is large relative to the period and semimajor axis of the binary. Global modelling of photometry and radial velocities indicates stellar masses of M-A = 0.426(-0.0049)(+0.0056)M(circle dot), M-B = 0.455(-0.0052)(+0.0058) M(circle dot )and stellar radii R-A = 0.461(-0.025)(+0.038)R(circle dot) , R-B = 0.411(-0.039)(+0.027)R(circle dot), respectively. Comparisons with stellar models for low-mass stars show that one star is consistent with model predictions whereas the other is substantially oversized. Spectral analysis of the system suggests a primary of spectral type M3V, consistent with both modelled masses and radii, and with spectral energy distribution fitting of NGTS photometry. As the most eccentric eclipsing M-dwarf binary known, NGTS J214358.5-380102 provides an interesting insight into the strength of tidal effects in the circularization of stellar orbits.