  We analyzed the Hipparcos astrometric observations of 47 stars that were
discovered to harbor giant planets and 14 stars with brown-dwarf secondary
candidates. The Hipparcos measurements were used together with the
corresponding stellar radial-velocity data to derive an astrometric orbit for
each system. To find out the significance of the derived astrometric orbits we
applied a "permutation" technique by which we analyzed the permuted Hipparcos
data to get false orbits. The size distribution of these false orbits indicated
the range of possibly random, false orbits that could be derived from the true
data. These tests could not find any astrometric orbit of the planet candidates
with significance higher than 99%, suggesting that most if not all orbits are
not real. Instead, we used the Hipparcos data to set upper limits on the masses
of the planet candidates. The lowest derived upper limit is that of 47UMa -
0.014 solar mass, which confirms the planetary nature of its unseen companion.
For 13 other planet candidates the upper limits exclude the stellar nature of
their companions, although brown-dwarf secondaries are still an option. These
negate the idea that all or most of the extrasolar planets are disguised
stellar secondaries. Of the 14 brown-dwarf candidates, our analysis reproduced
the results of Halbwachs et al., who derived significant astrometric orbits for
6 systems which imply secondaries with stellar masses. We show that another
star, HD164427, which was discovered only very recently, also has a secondary
with stellar mass. Our findings support Halbwachs et al. conclusion about the
possible existence of the "brown-dwarf desert" which separates the planets and
the stellar secondaries.
