2020MNRAS.494.3950A

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^+0.0014_–0.0037 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 ^+0.0056_–0.0049 M_☉, M_B = 0.455 ^+0.0058_–0.0052 M_☉ and stellar radii R_A = 0.461 ^+0.038_–0.025 R_☉, R_B = 0.411 ^+0.027_–0.039 R_☉, 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.