Binary orbits and masses (Söderhjelm):

Re-analysis of visual and speckle binary data, in combination with Hipparcos measurements, has yielded improved orbits, mass sums and individual masses for over 200 systems.  The relative uncertainties in the individual masses are < 7.5% for 42 main-sequence stars, and < 15% for 146 stars.  Compared with mass determinations from eclipsing binaries, this sample has the advantage that the components are widely separated, which excludes evolutionary interaction and permits strict comparison with theoretical models of isolated stars. Statistical modelling of the observed characteristics of binaries in the Hipparcos Catalogue has allowed to obtain new information about their true distributions, in particular concerning the mass ratios and semi-major axes. The prospects for binary observations by Gaia are spectacular.  Statistical modelling shows that some 50,000 mass values will be determined, of which some 10,000 could have relative errors less than 1%.
Main references: Söhjelm (A&A 341, 121, 1999), Söhjelm (ASPC 318, 413, 2004), Söhjelm (A&A 463, 683, 2007).

Hipparcos Transit Data (Quist, Lindegren):

Analysis techniques using the intermediate Hipparcos Transit Data (published on CD-ROM as part of the Hipparcos and Tycho Catalogues; ESA SP-1200, 1997) were further developed. The Transit Data are intermediate astrometric and photometric results permitting re-analysis of double and multiple stars, and which can combined in a rigorous manner with independent (ground-based) information, such as speckle data.  Programs to facilitate the use of the Transit Data have been made available on the Web. More information
Main reference: Quist & Lindegren (A&AS 138, 327, 1999).

Hipparcos binary statistics (Quist, Lindegren):

The statistics of binaries in the Hipparcos Catalogue were investigated by means of a model describing the spatial and orbital distributions of the objects as well as the observation process and Hipparcos double-star reductions.  In a sample of 13,000 bright main-sequence stars (the typical member being an A star at 100 pc distance from the Sun), the observed number of acceleration solutions leads to an estimated 0.25 ± 0.05 companion per star in the semi-major axis interval a = 1 to 10 AU. This is roughly a factor two higher than the companion frequency among nearby solar-type stars.  The results suggest a higher frequency of binaries among early main-sequence field stars, and a more peaked distribution in log(a), than among late-type field stars.
Main reference: Quist & Lindegren (A&A 361, 770, 2000).

Brown-dwarf detection from Gaia (Quist):

We simulated Gaia observations of brown-dwarf companions (masses 0.015 to 0.08 M_Sun) to main-sequence stars within 100 pc from the Sun. Such companions can be detected mainly from the non-linear proper motion that they induce in the parent star, but sometimes also by direct imaging. Gaia will be able to detect most such companions with periods from 0.03 to 200 years. For periods below 10 years, the complete orbits of the stars around the mass centre can be determined, giving a good handle of the companion masses without the sin(i) ambiguity in spectroscopic determinations.
Main reference: Quist (A&A 370, 672, 2001).