The very young brown dwarf candidate Cha H \alpha 8 was recently discovered to have a close ( \sim 1 AU ) companion by means of radial velocity monitoring .
We present additional radial velocity data obtained with UVES/VLT between 2007 and 2010 , which significantly improve the orbit determination of the system .
The combined data set spans ten years of radial velocity monitoring for Cha H \alpha 8 .
A Kepler fit to the data yields an orbital period of 5.2 years , an eccentricity of e =0.59 , and a radial velocity semi-amplitude of 2.4 km s ^ { -1 } .
A companion mass M _ { 2 } \sin i ( which is a lower limit due to the unknown orbital inclination i ) of 25 \pm 7 M _ { \mathrm { Jup } } and of 31 \pm 8 M _ { \mathrm { Jup } } is derived when using model-dependent mass estimates for the primary of 0.07 M _ { \odot } and 0.10 M _ { \odot } , respectively .
Assuming random orientation of orbits in space , we find a very high probability that the companion of Cha H \alpha 8 is of substellar nature : With a greater than 50 % probability ( i \geq 60 ^ { \circ } ) , the companion mass is between 30 and 35 M _ { \mathrm { Jup } } and the mass ratio M _ { 2 } / M _ { 1 } smaller than 0.4 ; with a greater than 87 % probability ( i \geq 30 ^ { \circ } ) the companion mass is between 30 and 69 M _ { \mathrm { Jup } } and the mass ratio smaller than 0.7 .
The absence of any evidence of the companion in the cross-correlation function together with the size of the radial velocity amplitude also indicate a mass ratio of at most 0.7 , and likely smaller .
Furthermore , the new data exclude the possibility that the companion has a mass in the planetary regime ( \leq 13 M _ { \mathrm { Jup } } ) .
We show that the companion contributes significantly to the total luminosity of the system : model-dependent estimates provide a minimum for the luminosity ratio L _ { 2 } / L _ { 1 } of 0.2 .
Cha H \alpha 8 is the fourth known spectroscopic brown dwarf or very low-mass stellar binary with determined orbital parameters , and the second known very young one .
With an age of only about 3 Myr , it is of particular interest to very low-mass formation and evolution theories .
In contrast to most other spectroscopic binaries , it has a relatively long orbital period and it might be possible to determine the astrometric orbit of the primary and , thus , the orbital inclination .