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 Astronomical Units ) .
We have found that Gl 569 B is itself double with a separation of only 0 ” .101 \pm 0 ” .002 ( 1 Astronomical Unit ) .
This detection demonstrates the superb spatial resolution that can be achieved with adaptive optics at Keck .
The difference in brightness between Gl 569 B and the companion is \sim 0.5 magnitudes 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 Gyr–1.0 Gyr ) and total mass of the system ( 0.09 M _ { \odot } –0.15 M _ { \odot } ) , we estimate that the orbital period is \sim 3 years .
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 .