The Hipparcos transit data: What, why and how?

The Hipparcos Transit Data are a collection of partially reduced, fully cal ibrated observations of (mostly) double and multiple stars obtained with th e ESA Hipparcos astrometry satellite. The data are publicly available, as p art of the CD-ROM set distributed with the Hipparcos and Tycho Catalogues ( ESA SP-1200, 1997), for about a third of the Hipparcos Catalogue entries in cluding all confirmed or suspected non-single stars. The Transit Data consi st of signal modulation parameters derived from the individual transits of the targets across the Hipparcos focal grid. The Transit Data permit re-red uction of the satellite data for individual objects, using arbitrarily comp lex object models in which time-variable photometric as well as geometric c haracteristics may be taken into account. We describe the structure and con tents of the Transit Data files and give examples of how the data can be us ed. Some of the applications use standard astronomical software: Difmap or AIPS for aperture synthesis imaging, and GaussFit for detailed model fittin g. Fortran code converting the data into formats suitable fur these applica tion programs has been made public in order to encourage and facilitate the use of Hipparcos Transit Data.