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 August 1999 , we are able to see orbital motion and to determine the orbital parameters of the pair .
We find the orbital period to be 892 \pm 25 days , the semi-major axis to be 0.90 \pm 0.02 AU , the eccentricity to be 0.32 \pm 0.02 and the inclination of the system to be 34 \pm 3 degrees ( 1- \sigma ) .
The total mass is found to be 0.123 _ { -0.022 } ^ { +0.027 } M _ { \odot } ( 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 M8.5V ( Gl 569Ba ) and M9V ( Gl 569Bb ) with an uncertainty of half a subclass .
We also present new J - and K -band photometry which allows us to accurately place the objects in the HR diagram .
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 .