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[abridged] We attempt to improve on the characterization of the properties(orbital elements, masses) of two Doppler-detected sub-stellar companions tothe nearby G dwarfs HD 131664 and HD 43848. We carry out orbital fits to theHipparcos IAD for the two stars, taking advantage of the knowledge of thespectroscopic orbits, and solving for the two orbital elements that can bedetermined in principle solely by astrometry, the inclination angle $i$ and thelongitude of the ascending node $\Omega$. A number of checks are carried out inorder to assess the reliability of the orbital solutions thus obtained. Thebest-fit solution for HD 131664 yields $i=55\pm33$ deg and $\Omega=22\pm28$deg. The resulting inferred true companion mass is then $M_c = 23_{-5}^{+26}$$M_J$. For \object{HD 43848}, we find $i=12\pm7$ deg and $\Omega=288\pm22$ deg,and $M_c = 120_{-43}^{+167}$ $M_J$. 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 adistribution of false astrometric orbits obtained for single stars observed byHipparcos, the astrometric signal of the two companions to HD 131664 and HD43848 is then considered detected in the Hipparcos IAD, with a level ofstatistical confidence not exceeding 95%. We constrain the true mass of HD131664b to that of a brown dwarf to within a somewhat statistically significantdegree of confidence ($\sim2-\sigma$). For HD 43848b, a true mass in the browndwarf regime is ruled out at the $1-\sigma$ confidence level. [abridged]

Hipparcos preliminary astrometric masses for the two close-in companions to HD 131664 and HD 43848. A brown dwarf and a low mass star