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 characterization of an eclipsing binary identified by the Next Generation Transit Survey in the ∼115-Myr-old Blanco 1 open cluster. NGTS J0002−29 comprises three M dwarfs: a short-period binary and a companion in a wider orbit. This system is the first well-characterized, low-mass eclipsing binary in Blanco 1. With a low mass ratio, a tertiary companion, and binary components that straddle the fully convective boundary, it is an important benchmark system, and one of only two well-characterized, low-mass eclipsing binaries at this age. We simultaneously model light curves from NGTS, TESS, SPECULOOS, and SAAO, radial velocities from VLT/UVES and Keck/HIRES, and the system’s spectral energy distribution. We find that the binary components travel on circular orbits around their common centre of mass in P_orb = 1.098 005 24 ± 0.000 000 38 d, and have masses M_pri = 0.3978 ± 0.0033 M_⊙ and M_sec = 0.2245 ± 0.0018 M_⊙, radii R_pri = 0.4037 ± 0.0048 R_⊙ and R_sec = 0.2759 ± 0.0055 R_⊙, and effective temperatures T_pri=3372⁺⁴⁴₋₃₇ K and T_sec=3231⁺³⁸₋₃₁ K. We compare these properties to the predictions of seven stellar evolution models, which typically imply an inflated primary. The system joins a list of 19 well-characterized, low-mass, sub-Gyr, stellar-mass eclipsing binaries, which constitute some of the strongest observational tests of stellar evolution theory at low masses and young ages.