1 Introduction

The data used in the present study have been extracted from the master files for StarGuides (Heck 1997a) and StarWorlds (Heck et al. 1994), the latter one being the WWW version of the former one which is a classical directory on paper. The web version is itself the follow-on of on-line yellow-page services established previously at ESA/ESRIN (StarWays - see Heck et al. 1992) and at ESO (StarGates - see Albrecht & Heck 1992). It is also one of the three components of the StarPages (see e.g. Heck 1997b) including in addition a database of personal web pages (StarHeads - see Heck 1995) and a huge dictionary of acronyms focused on astronomy, space and related fields and activities ( StarBits - see Heck et al. 1994).

The expertise built up over now almost a quarter of a century in this kind of compilations and services (involving authentication and quality checking schemes together with a permanent updating process), as well as the overall stability of the master files, guarantee an excellent exhaustivity of the entries and an homogeneous coverage of the practical data gathered together. They are certainly the best sources available today for the study at hand. It is appropriate to recall here that, contrary to most on-line resources, StarWorlds is not only WWW-oriented.