Dynamical mass determination for the very low mass stars LHS 1070 B and C

We followed the relative orbit of the close pair of very late-type stars in the nearby triple system LHS 1070 during the past seven years. These observations are used to derive the astrometric elements of the orbit. Using the period of 16.1 ± 1.4 years and assuming the distance of 7.39 pc given by van Altena (1995) to be exactly correct, the combined mass of components B and C is 0.138 ± 0.003 M_⊙. Otherwise, this mass scales with the third power of the distance, which presently is not known to better than 9%. The dynamical mass would equal the combined mass of 0.161 ± 0.002 M_⊙ derived from the theoretical mass-luminosity relation of Baraffe et al. (1998) and Chabrier et al. (2000) for a 5% larger distance of 7.78 pc. A critical comparison with the theoretical relations therefore has to await more precise parallax determinations for this comparatively faint system with visual magnitude V = 15.3. The wide orbit of component A around the center of mass of components B and C cannot yet be determined from our measurements. However, we can show that it is probably co-planar with the close pair orbit and should have a period in the range 60-1000 years.