Zero-point and external errors of HIPPARCOS parallaxes

A good knowledge of both accuracy and precision of the Hipparcos parallaxes is one of the keys for their future scientific use. For this purpose, the Hipparcos preliminary parallaxes, as obtained after the processing of the first 30 months of Hipparcos data, are compared to various ground-based parallax estimates, using astrometric, photometric and spectroscopic data. In order to find unbiased values of the global zero-point and of external errors, a new maximum-likelihood algorithm has been built, taking into account the censorships of the observed data. Applying the method to a sample of distant stars, it is shown that the global zero-point error of the Hipparcos preliminary parallaxes should be smaller than 0.1mas and that the external errors are unlikely to be underestimated by more than about 5%.