NGTS clusters survey – III. A low-mass eclipsing binary in the Blanco 1 open cluster spanning the fully convective boundary

We present the discovery and characterisation of an eclipsing binary identified by Next Generation Transit Survey in $\sim$115 Myr old Blanco 1 open cluster. NGTS J0002-29 comprises three M dwarfs: a short-period companion wider orbit. This system is first well-characterised, low-mass 1. With low mass ratio, tertiary components that straddle fully convective boundary, it important benchmark system, one only two binaries at this age. simultaneously model light curves from NGTS, TESS, SPECULOOS SAAO, radial velocities VLT/UVES Keck/HIRES, system's spectral energy distribution. find travel on circular orbits around their common centre $P_{\rm orb} = 1.09800524 \pm 0.00000038$ days, have masses $M_{\rm pri}=0.3978\pm 0.0033$ M$_{\odot}$ sec}=0.2245\pm 0.0018$ M$_{\odot}$, radii $R_{\rm pri}=0.4037\pm 0.0048$ R$_{\odot}$ sec}=0.2759\pm 0.0055$ R$_{\odot}$, effective temperatures $T_{\rm pri}=3372\,^{+44}_{-37}$ K sec}=3231\,^{+38}_{-31}$ K. compare these properties to predictions seven stellar evolution models, which typically imply inflated primary. The joins list 19 low-mass, sub-Gyr, stellar-mass binaries, constitute some strongest observational tests theory young ages.