Context : Several mechanisms for forming brown dwarfs have been proposed , which are today believed not to be mutually exclusive .
Among the fundamental characteristics of brown dwarfs which are intrinsically tied to their origin , multiplicity is of particular relevance .
Any successful determination of the actual mass for such objects in systems is thus worthwhile as it allows one to improve on the characterization of the multiplicity properties ( e.g , frequency , separation , mass-ratio distribution ) of sub-stellar companions .

Aims : We attempt to improve on the characterization of the properties ( orbital elements , masses ) of two Doppler-detected sub-stellar companions to the nearby G dwarfs HD 131664 and HD 43848 .

Methods : We carry out orbital fits to the Hipparcos Intermediate Astrometric Data ( IAD ) for the two stars , taking advantage of the knowledge of the spectroscopic orbits , and solving for the two orbital elements that can be determined in principle solely by astrometry , the inclination angle i and the longitude of the ascending node \Omega .
A number of checks are carried out in order to assess the reliability of the orbital solutions thus obtained .

Results : The best-fit solution for HD 131664 yields i = 55 \pm 33 deg and \Omega = 22 \pm 28 deg .
The resulting inferred true companion mass is then M _ { c } = 23 _ { -5 } ^ { +26 } M _ { J } .
For HD 43848 , we find i = 12 \pm 7 deg and \Omega = 288 \pm 22 deg , and a corresponding M _ { c } = 120 _ { -43 } ^ { +167 } M _ { J } .
Based on the statistical evidence from an F -test , the study of the joint confidence intervals of variation of i and \Omega , and the comparison of the derived orbital semi-major axes with a distribution of false astrometric orbits obtained for single stars observed by Hipparcos , the astrometric signal of the two companions to HD 131664 and HD 43848 is then considered detected in the Hipparcos IAD , with a level of statistical confidence not exceeding 95 % .

Conclusions : We constrain the true mass of HD 131664 b to that of a brown dwarf to within a somewhat statistically significant degree of confidence ( \sim 2 - \sigma ) .
For HD 43848 b , a true mass in the brown dwarf regime is ruled out at the 1 - \sigma confidence level .
The results are discussed in the context of the properties of the ( few ) close sub-stellar and massive planetary companions to nearby solar-type stars and their implications for proposed models of formation and structure of massive planets and brown dwarfs .