Abstract

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 MA = 0.426+0.0056 -0.0049 M⊙, MB = 0.455+0.0058 -0.0052 M⊙ and stellar radii RA = 0.461+0.038 -0.025 R⊙, RB = 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 bothmodelledmasses 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.