The orbit of the brown dwarf binary gliese 569B

We present photometric, astrometric, and spectroscopic observations of the nearby (9.8 pc) low-mass binary G1 569Bab (in turn being a companion to the early-M star Gl 569A) made with the Keck adaptive optics facility. Having observed Gl 569Bab since 1999 August, we are able to see orbital motion and to determine the orbital parameters of the pair. We find the orbital perio d to be 892 +/- 25 days, the semimajor axis to be 0.90 +/-0.02 AU, the ecce ntricity to be 0.32 +/-0.02, and the inclination of the system to be 34 deg rees +/-3 degrees (1 sigma). The total mass is found to be 0.123(-0.022)(+0 .027) M-circle dot (3 sigma). In addition, we have obtained low-resolution (R=1500-1700) near-infrared spectra of each of the components in the J and K bands. We determine the spectral types of the objects to be M8.5 V (Gl 56 9Ba) and M9 V (Gl 569Bb) with an uncertainty of half a subclass. We also pr esent new J- and K-band photometry that allows us to place the objects in t he H-R diagram accurately. Most likely the binary system is comprised of tw o brown dwarfs with a mass ratio of 0.89 and with an age of approximately 3 00 Myr.