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 Gl 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 period to be 892 ± 25 days, the semimajor axis to be 0.90 ± 0.02 AU, the eccentricity to be 0.32 ± 0.02, and the inclination of the system to be 34^° ± 3^° (1 σ). The total mass is found to be 0.123 M_☉ (3 σ). 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 569Ba) and M9 V (Gl 569Bb) with an uncertainty of half a subclass. We also present new J- and K-band photometry that allows us to place the objects in the H-R diagram accurately. Most likely the binary system is comprised of two brown dwarfs with a mass ratio of 0.89 and with an age of approximately 300 Myr.