[abridged] We report four years of radial velocity monitoring observations of SDSS J080531.84+481233.0 that reveal significant and periodic variability, confirming the binary nature of the source. We infer an orbital period of 2.02$\pm$0.03 yr, a semi-major axis of 0.76$^{ +0.05}_{ -0.06}$ AU, and an eccentricity of 0.46$\pm$0.05, consistent with the amplitude of astrometric variability and prior attempts to resolve the system. Folding in constraints based on the spectral types of the components (L4$\pm$0.7 and T5.5$\pm$1.1), corresponding effective temperatures, and brown dwarf evolutionary models, we further constrain the orbital inclination of this system to be nearly edge-on (90$^o\pm$19$^o$), and deduce a large system mass ratio (M$_2$/M$_1$ = 0.86$^{ +0.10}_{ -0.12}$), substellar components (M$_1$ = 0.057$^{ +0.016}_{ -0.014}$ M$_{ \odot}$, M$_2$ = 0.048$^{ +0.008}_{ -0.010}$ M$_{ \odot}$), and a relatively old system age (minimum age = 4.0$^{ +1.9}_{ -1.2}$ Gyr). The measured projected rotational velocity of the primary ($v\sin{ i}$ = 34.1$\pm$0.7 km/s) implies that this inactive source is a rapid rotator (period $\lesssim$ 3 hr) and a viable system for testing spin-orbit alignment in very-low-mass multiples. The combination of well-determined component atmospheric properties and masses near and/or below the hydrogen minimum mass make SDSS J0805+4812AB an important system for future tests of brown dwarf evolutionary models.
Categories: astro-ph.SR