J1250+0455AB an ultracool binary in a hierarchical triple system

We report the discovery of the ultracool dwarf binary system J1250+0455AB, a low-mass (M0.2 M) system in which the components straddle the M/L dwarf boundary. The binary was resolved through near-infrared adaptive optics imaging with LUCI1-SOUL on the Large Binocular Telescope, revealing a projected angular separation of 0.17 0.015 arcsec, which, combined with a system distance of pc, corresponds to a physical separation of 12.2 1.5 AU at a position angle of 84.8 0.2. We estimated the orbital period of J1250+0455AB to be 156 8 yr, the bolometric luminosities of the primary and secondary luminosities as and, respectively, with the spectral types of M9 and L0 determined through binary template fitting and spectrophotometric relations. This binary system is part of a hierarchical triple with a separation of 10.44 arcsec from its primary. We estimated the age of the system from the rotational period of the primary star as Gyr. Using evolutionary models, for each component we estimate the mass [0.079 0.002 M/ 0.072 0.003 M], effective temperature [2350 38 K / 2200 43 K], and radius [0.113 0.003 R/ 0.108 0.002 R]. Based on the system’s binding energy, total mass, and separation, J1250+0455AB is predicted to be a highly stable system, remaining bound for 10 Gyr. J1250+0455AB extends the growing population of UCD benchmark systems, providing a new system for refining evolutionary theories at the lowest stellar masses into the substellar regime.