Abstract

We present the discovery and characterization of an eclipsing binary identified by the Next Generation Transit Survey in the similar to 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-circle dot and M-sec = 0.2245 +/- 0.0018 M-circle dot, radii R-pri = 0.4037 +/- 0.0048 R-circle dot and R-sec = 0.2759 +/- 0.0055 R-circle dot, and effective temperatures T-pri = 3372(-37)(+44) K and T-sec = 3231(-31)(+38) 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.