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 obser vations 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 po wer of the distance, which presently is not known to better than 9%. The dy namical mass would equal the combined mass of 0.161 +/- 0.002 M. derived fr om the theoretical mass-luminosity relation of Baraffe et al. (1998) and Ch abrier ct al. (2000) for a 5% larger distance of 7.78 pc. A critical compar ison with the theoretical relations therefore has to await more precise par allax determinations for this comparatively faint system with visual magnit ude V = 15.3. The wide orbit of component A around the center of mass of co mponents B and C cannot yet be determined from our measurements. Hers-ever, we can show that it is probably co-planar with the close pair orbit and sh ould have a period in the range 60-1000 years.