The study of double stars has since long been recognized as a basic key to the understanding of star formation and stellar evolution. Moreover, close visual double stars have always been systematically neglected in photometric observational programmes although they contain an important part of physically associated systems. It is then timely to organize major observational programmes of these objects for a number of good reasons:

1. 1. The frequency of double stars is continuously reviewed and the rate of their detection is steadily increasing — both from ground–based and space observations — in such a way that they no longer can be discarded in any models of galactic structure;

2. 2. Space observations (HIPPARCOS, HST) significantly improve the quality and the importance of stellar samples and permit to better take into account some of the selection effects;

3. 3. The high–quality astrometric (and also photometric) data that will be provided for such systems by the space observations should be matched with accurate and homogeneous complementary astrophysical information such as colour indices and spectral classification. Such information for close visual double stars is unfortunately almost non-existent but are now being more easily accessible with the use of CCD detectors.

The best empirical evolutionary tracks are provided by open cluster HR diagrams. The scarcity of giants in young clusters implies that diagrams of clusters having similar ages and metallicities have to be summed. Using UBV data, Mermilliod (1981a) has divided the nearby open clusters into 14 age groups, and has described the properties of the composite diagrams MV/(B-V) and Mbol/log T. Several features complicate the structure of composite HRD, namely the presence of double stars which often mimic extended blue loops and the contamination by non-member stars difficult to identify as such in distant clusters. In order to refine the red giant observed evolutionary properties, cluster giants have been remeasured in the Geneva photometry and with the spectrovelocimeter CORAVEL.

When we began to collect systematically photometric data it was felt that we needed an appropriate system of identification for our catalogues on magnetic tape. The current system of identification which uses catalogue or list name and star number is best suitable for printed catalogues but brings out many drawbacks when using and intersecting catalogues on magnetic tapes. Thus we developed a coded system of identification with the two basic conditions that it allows to include in the files stars with all possible Kinds of numbering found in the literature, and that computer use was as simple as possible (Mermilliod, 1973).