We present the discovery of NGTS J214358.5-380102 , an eccentric M-dwarf binary discovered by the Next Generation Transit Survey .
The system period of 7.618 days 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 semi-major axis of the binary .
Global modelling of photometry and radial velocities indicate stellar masses of M _ { A } = 0.426 ^ { +0.0056 } _ { -0.0049 } M _ { \odot } , M _ { B } = 0.455 ^ { +0.0058 } _ { -0.0052 } M _ { \odot } and stellar radii R _ { A } = 0.461 ^ { +0.038 } _ { -0.025 } R _ { \odot } , R _ { B } = 0.411 ^ { +0.027 } _ { -0.039 } R _ { \odot } , 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 SED fitting of NGTS photometry .
As the most eccentric M-dwarf binary known , NGTS J214358.5-380102 provides an interesting insight into the strength of tidal effects in the circularisation of stellar orbits .