A low-mass eclipsing binary within the fully convective zone from the NGTS

Abstract

We have discovered a new, near-equal mass, eclipsing M dwarf binary from the Next Generation Transit Survey. This system is only one of 3 field age ($>$ 1 Gyr), late M dwarf eclipsing binaries known, and has a period of 1.74774 days, similar to that of CM~Dra and KOI126. Modelling of the eclipses and radial velocities shows that the component masses are $M_{\mathrm{pri}}$=0.17391${}_{0.00099}^{+0.00153}$ $M_{\odot }$, $M_{\sec }$=0.17418${}_{-0.00059}^{+0.00193}$ $M_{\odot }$; radii are $R_{\mathrm{pri}}$=0.2045${}_{-0.0058}^{+0.0038}$ $R_{\odot }$, $R_{\sec }$=0.2168${}_{-0.0048}^{+0.0047}$ $R_{\odot }$. The effective temperatures are $T_{\mathrm{pri}}=2995{}_{-105}^{+85}$ K and $T_{\sec }=2997{}_{-101}^{+66}$ K, consistent with M5 dwarfs and broadly consistent with main sequence models. This pair represents a valuable addition which can be used to constrain the mass-radius relation at the low mass end of the stellar sequence.

12 pages, 9 Figures, Accepted for publication in MNRAS