Stars in galactic open clusters have been carefully selected for Hipparcos observations (Mermilliod & Turon 1989), and some of the closest clusters are nicely sampled. In addition to detailed studies of very nearby clusters, mean distances with relative precision better than 10% can be determined from Hipparcos observations for about 10 clusters of various ages and metallicities, and for 10 other clusters with relative precision between 10 and 20%. A completely new insight in main-sequence fitting is being obtained from these studies.
Besides the Hyades cluster, studies have been performed on the
Pleiades (van Leeuwen & Hansen Ruiz 1997, Hansen Ruiz & van Leeuwen
1997, Mermilliod et al. 1997); on Coma Ber, IC 2602, IC 2391,
Praesepe, 
Per and Blanco 1 (Mermilliod et al. 1997); on
Praesepe, IC 4756, NGC 6475, NGC 6633, Stock 2, NGC 2516 and NGC 3532
(Robichon et al. 1997). The mean distances of all these clusters have
been determined from Hipparcos intermediate data of member stars,
taking into account the correlations between the abscissae of stars
within a few degrees of the sky (such as in clusters) due to the
operation mode of the satellite.
The various authors agree on the mean distance of the Pleiades cluster:
pc, or 
(Mermilliod et al. 1997),
(van Leeuwen & Hansen Ruiz 1997). This
distance is some 15 per cent smaller than that consistently
derived in earlier determinations. There is no way to reconcile
this distance with earlier photometric determinations.
The comparison of the main sequence positions of the other open
clusters in the HR diagram is in contradiction with the commonly
accepted interpretation of metallicity effects: Hyades, Praesepe, Coma
Ber, Persei and Blanco 1 define the same main sequence despite
their differences in metallicity. The Pleiades, and
probably IC 2391 and IC 2602, define a sequence about 0.5 mag fainter
that the previous one (Mermilliod et al. 1997). NGC 2516 defines a
sequence 0.7 mag below Praesepe, much larger than the difference
of 0.25 mag due to metallicity obtained when using usual helium
variations (Robichon et al. 1997).
The parameter responsible for these differences has not yet been identified, although they could be accounted for by a significant increase in the Helium abundance. However there is presently no indication of a much higher Helium abundance in nearby clusters. This unexpected result should be understood before making any further main-sequence fitting of more distant cluster.