A low-mass eclipsing binary within the fully convective zone from the Next Generation Transit Survey

We have discovered a new, near-equal-mass, eclipsing M dwarf binary from the Next Generation Transit Survey. This system is only one of three field-age (>1 Gyr), late M dwarf eclipsing binaries known, and has a period of 1.74774 d, similar to that of CM Dra and KOI126. Modelling of the eclipses and radial velocities shows that the component masses are M_pri =0.17391 ^{+0.00153}_{-0.00099} M_⊙, M_sec = 0.17418 ^{+0.00193}_{-0.00059} M_⊙; and the radii are R_pri = 0.2045 ^{+0.0038}_{-0.0058} R_⊙, R_sec= 0.2168 ^{+0.0047}_{-0.0048} R_⊙. The effective temperatures are T_pri=2995 ^{+85}_{-105} K and T_sec=2997 ^{+66}_{-101}K, consistent with M5 dwarfs and broadly consistent with main-sequence models. This pair represents a valuable addition that can be used to constrain the mass-radius relationship at the low-mass end of the stellar sequence.