Hipparcos distance calibrations for 9 open clusters

Parallaxes have been determined for 9 nearby open clusters in order to derive a composite, age resolved, HR diagram. An age-luminosity relation was found to exist for main-sequence stars. A suggested 1 mas error in the Hipparcos parallax for the Pleiades, to explain the difference with the ground-based photometric distance estimate, seems unlikely: 5 other young open clusters examined (IC 2602, IC 2391, cu Per, NGC 2451 and Blanco 1) all share the Pleiades main sequence locus in the HR diagram. The 3 older open clusters (Coma Per, Praesepe and NGC 6475) were found displaced towards brighter magnitudes. These trends are more pronounced than predicted by theoretical stellar evolution models, and affect distance, metallicity and possibly also age determinations for open- and globular clusters through the use of theoretical isochrones.