The pre-main sequence star CHXR 74 ( M4.25 ) in Chamaeleon I was found a few years ago to be a very low-mass spectroscopic binary .
A determination of its mass would provide a valuable dynamical mass measurement at young ages in the poorly constrained mass regime of < 0.3 { M _ { \odot } } .
We carried out follow-up radial velocity monitoring with UVES/VLT between 2008 and 2011 and high-resolution adaptive-optic-assisted imaging with NACO/VLT in 2008 with the aim of constraining the binary orbit .
We present an orbital solution of the system based on the combined radial velocity data set , which spans more than eleven years of UVES monitoring for CHXR 74 .
The best-fit Kepler model has an orbital period of 13.1 years , zero eccentricity , and a radial velocity semi-amplitude of 2.2 km s ^ { -1 } .
A companion mass M _ { 2 } \sin i ( which is a lower limit due to the unknown orbital inclination i ) of 0.08 { M _ { \odot } } is derived by using a model-dependent mass estimate for the primary of 0.24 { M _ { \odot } } .
The binary separation ( a _ { 1 } \sin i + a _ { 2 } ) for an inclination of 90 ^ { \circ } is 3.8 AU , which corresponds to 23 mas .
Complementary NACO/VLT images of CHXR 74 were taken with the aim to directly resolve the binary .
While there are marginal signs of an extended point spread function ( PSF ) , we have detected no convincing companion to CHXR 74 in the NACO images .
From the non-detection of the companion together with a prediction of the binary separation at the time of the NACO observations , we derive an upper limit for the K -band brightness ratio of the two binary components of 0.5 .
This allows us to estimate an upper limit of the companion mass of 0.14 { M _ { \odot } } by applying evolutionary models .
Thus , we confirm that CHXR 74 is a very low-mass spectroscopic binary and constrain the secondary mass to lie within the range of about 0.08 and 0.14 { M _ { \odot } } .
We predict an astrometric signal of the primary between 0.2 and 0.4 mas when taking into account the luminosity of the companion .
The GAIA astrometric mission might well be able to solve the astrometric orbit of the primary and in combination with the presented radial velocity data determine an absolute companion mass .