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2 Cluster selection

The basic data of the open clusters (positions, distances, magnitudes of brightest stars) were taken from the catalogue of Lyngå (1987). 1151 open clusters are listed in his catalogue, most of which however are too faint for Hipparcos. We therefore removed all clusters that have never been studied at all, since they are presumably very faint and even if they contain stars bright enough for Hipparcos, there is no information available to identify them as members. In addition, we omitted clusters where, according to Lyngå (1987), brightest members are fainter than V = 12 mag, since it is very unlikely that they contain Hipparcos stars. Nearby clusters with distances D < 200 pc were also omitted from our analysis. This was done because they have large angular diameters, so that the assumption of a common proper motion of all cluster stars is not valid for them. They require a different kind of analysis (for example convergent point methods like in Perryman et al. 1998), which is beyond the scope of the present paper. Nearby clusters not discussed here include the Hyades, Pleiades, Coma Ber, IC 2391, IC 2602, $\alpha$ Persei Cluster, Praesepe (NGC 2632) and the UMA Star Cluster. Hipparcos results for most of them can already be found in the literature (Perryman et al. 1998; van Leeuwen 1999; Robichon et al. 1999). Our final list contained 360 clusters which may have members in the Hipparcos Catalogue.

We note that Lyngå (1987) lists two additional clusters with distances less than 200 pc (Col 399, Upg 1), but Baumgardt (1998) has recently shown on the basis of proper motions from the Hipparcos and ACT (Urban et al. 1997) catalogues that they do not exist at all. We finally note that Platais et al. (1998) found several new cluster candidates in the Hipparcos Catalogue, but most of them require further study to confirm their reality.


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