CCD photometry and astrometry for visual double and multiple stars of the HIPPARCOS catalogue ? I. Presentation of the large scale project

A description is given of the activities of an inter- national working group created with the aim of obtaining both photometric and astrometric observations of visual double and multiple stars with angular separations in the range of one to fifteen arcseconds, that formed part of the HIPPARCOS Input Catalogue. The scientific aims and realisations of this European network are given. About 50 observational missions have been carried out in both hemispheres according to a pre-defined proto- col. We describe the general and specifically designed methods used for the reduction of large amounts of CCD observations of double stars and give an outline of the results already presented and soon to be expected.