NGTS J214358.5-380102 - NGTS discovery of the most eccentric known eclipsing 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 $0.323^{+0.0014}_{-0.0037}$ 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.