2000ApJ...529L..37M

We report observations obtained with the Keck adaptive optics facility of the nearby (d=9.8 pc) binary Gl 569. The system was known to be composed of a cool primary (dM2) and a very cool secondary (dM8.5) with a separation of 5" (49 AU). We have found that Gl 569B is itself double with a separation of only 0".101±0".002 (1 AU). This detection demonstrates the superb spatial resolution that can be achieved with adaptive optics at Keck. The difference in brightness between Gl 569B and the companion is ∼0.5 mag in the J, H, and K' bands. Thus, both objects have similarly red colors and very likely constitute a very low mass binary system. For reasonable assumptions about the age (0.12-1.0 Gyr) and total mass of the system (0.09-0.15 M_☉), we estimate that the orbital period is ∼3 yr. Follow-up observations will allow us to obtain an astrometric orbit solution and will yield direct dynamical masses that can constrain evolutionary models of very low mass stars and brown dwarfs.