2025MNRAS.542..310B

We report the discovery of the ultracool dwarf binary system J1250+0455AB, a low-mass (M$_\odot$$_\mathrm{tot} <$ 0.2 M$_\odot$) 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 $\pm$ 0.015 arcsec, which, combined with a system distance of $71 \pm 5.8$ pc, corresponds to a physical separation of 12.2 $\pm$ 1.5 AU at a position angle of 84.8 $\pm$ 0.2$^{\circ }$. We estimated the orbital period of J1250+0455AB to be 156 $\pm$ 8 yr, the bolometric luminosities of the primary and secondary luminosities as $\log (L_\mathrm{bol} / L_\odot) = -3.45 \pm 0.04$ and $-3.58 \pm 0.04$, 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 $0.56^{+0.07}_{-0.06}$ Gyr. Using evolutionary models, for each component we estimate the mass [0.079 $\pm$ 0.002 M$_\odot$ / 0.072 $\pm$ 0.003 M$_\odot$], effective temperature [2350 $\pm$ 38 K / 2200 $\pm$ 43 K], and radius [0.113 $\pm$ 0.003 R$_\odot$ / 0.108 $\pm$ 0.002 R$_\odot$]. 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.