Hipparcos astrometric orbits for two brown dwarf companions:HD 38529 and HD 168443

Context.HD 38529 and HD 168443 have previously been identified as systems with two substellar companion candidates using precise radial velocity measurements.Aims.We want to further constrain their orbits and the nature of the outer companions.Methods.We fit astrometric orbits of the outer substellar companions in the two systems to the Hipparcos Intermediate Astrometric Data.Results.The fit constrains all possible solutions to a small region in the parameter space of the two missing orbital parameters (inclination i and ascending node Ω). This can be interpreted as a possible real detection of the astrometric signatures of the companions in the Hipparcos data, although there is still a 14–18% chance that the signal is not detectable in the data, according to an F-test. However, even in the case of a non-detection of the companion signal in the astrometric data, the knowledge of the spectroscopic orbital parameters enables us to place tight constraints on these two missing parameters, so that the astrometric orbit is fully determined (with confidence levels of around 80% for HD 38529, 95% for HD 168443). Inclinations derived from these astrometric fits enable us to calculate masses for the substellar companions rather than lower or upper limits. The best fit solution for HD 38529, (i, Ω) = (160°, 52°), yields a mass of 37 MJup for the outer companion. For HD 168443, we derive best fit parameters of (i, Ω) = (150°, 19°), which imply a companion mass of MJup.Conclusions.The outer companions in both systems are thus brown dwarfs.