1 Introduction

The ESA astrometric satellite, Hipparcos, observed a list of $120\,000$ stars with a very high accuracy ([ESA 1997]). The stars were observed as they crossed the main grid in the focal plane of the telescope and the oscillating signal was recorded. In fact only a small patch of diameter 38 arcsec, centred on the predicted position of the star, was sensitive to light, still allowing the combined signal for double and multiple systems to be measured. The grid has a period of 1.2 arcsec, and therefore each observation can only measure an abscissa in the scan direction modulo 1.2 arcsec. For the vast majority of stars this scheme worked very fine because the positions were known a priori much better than this grid step ambiguity. Some several hundreds of stars, often components of double or multiple systems, suffered however from poorly known position and proper motion making the data reduction very difficult. In some cases no solution was reached at all or only a very poor solution, which was however accepted in the, by necessity, rather automatic processing.

The Hipparcos satellite also included a system of star mapper slits serving the task of attitude determination, but at the same time performing a survey of the entire sky. The analysis of the star mapper data resulted in the Tycho Catalogue ([ESA 1997]) of more than $1\,000\,000$ stars. Recently Høg et al. ([1998b]) have performed a much improved reduction of the same star mapper data leading to the Tycho-2 Catalogue ([Høg et al. 2000]) of $2\,500\,000$ stars, reaching fainter stars and achieving a better resolution of double stars.

Today, revised solutions are available for about 600 Hipparcos stars. The Hipparcos Catalogue itself contains revisions for 240 stars in the General Notes. The notes in question read "Investigations carried out after the main catalogue was finalised led to a more likely solution for this entry ...''. To these we can add new solutions for 139 stars by [Falin & Mignard (1999)] and the results given in the present paper.

In their investigation, [Falin & Mignard (1999)] used the unpublished data base of about $20\,000$ stars from the Fundamental Astrometry by Space Techniques Consortium (FAST) which was one of the two consortia carrying out the Hipparcos reductions. The other consortium, the Northern Data Analysis Corsortium (NDAC), has prepared a calibrated data base of Hipparcos observations of more than $37\,000$ targets, which is included in the published Hipparcos Catalogue ([ESA 1997], [Quist & Lindegren 1999]) as the "Hipparcos Transit Data''. We have been using these Transit Data in combination with the unpublished Tycho Data Base of Identified Counts for our rediscussion of 257 Hipparcos stars. Preliminary results for a handful of these systems have already been published elsewhere (Makarov & Fabricius [1999]).

The present paper presents results based on observations from the Hipparcos satellite, so in that sense, and in that sense only, is it Hipparcos astrometry. It should not be seen as an official part of the Hipparcos Catalogue.