A Very Low Mass Star in an Extreme Mass Ratio Eclipsing Binary from NGTS

We present the discovery of NGTS J0930-18, an extreme mass ratio eclipsing M-dwarf binary. The system consists of an early M-dwarf primary and a late M-dwarf secondary close to the hydrogen burning limit. We combine the NGTS discovery photometry with data from TESS and SAAO, as well as radial velocities from HARPS to characterise the system. We find that the secondary component (NGTS J0930-18B) has a mass of \(M_{*}= 0.0818 ^{0.0040}_{-0.0015} M_{\odot}\) and a radius of \(R_{*}= 0.1059 ^{0.0023}_{-0.0021} R_{\odot}\) making it one of the lowest mass stars with precise direct mass and radius measurements. 
NGTS J0930-18B lies in the sparsely populated mass-radius parameter space close to the hydrogen burning limit, making its characterisation important given the growing number of terrestrial planets being discovered orbiting low mass stars.
The system has a mass ratio \((\frac{M_{pri}}{M_{sec}})\) of \(q=0.1407^{0.0065}_{-0.017}\) making it the lowest mass ratio eclipsing M-M binary known, which poses interesting questions for evolutionary models that suggest low mass ratio M-dwarf binaries should be difficult to create.