  We present new BVRI photometry for a sample of 54 local G and K stars with
accurate Hipparcos parallaxes in the metallicity range -0.4 < [Fe/H] < +0.3. We
use this sample to develop a completely model-independent main sequence (MS)
fitting method which we apply to 4 open clusters - the Hyades, Praesepe, the
Pleiades and NGC 2516 - which all have direct Hipparcos parallax distance
determinations. Comparison of our MS-fitting results with distances derived
from Hipparcos parallaxes enables us to explore whether the discrepancy between
the Hipparcos distance scale and other MS-fitting methods found for some
clusters is a consequence of model assumptions. We find good agreement between
our results and the Hipparcos ones for the Hyades and Praesepe. For the
Pleiades and NGC 2516, when adopting the solar abundance determined from
spectroscopy, we find significant disagreement at a level similar to that found
by other MS-fitting studies. However, the colour-colour relationship for both
these clusters suggests that their metallicity is significantly subsolar. Since
the MS-fitting method relies on matching the cluster colours to a template MS,
we argue that, when applying this method, the appropriate metallicity to adopt
is the photometric subsolar one, not the solar abundance indicated by
spectroscopy. Adopting photometric metallicities for all 4 clusters, we find
complete agreement with the Hipparcos results and hence we conclude that the
mismatch between the spectroscopic and photometric abundances for the Pleiades
and NGC 2516 is responsible for the discrepancies in distance estimates found
by previous studies. The origin of this mismatch in abundance scales remains an
unsolved problem and some possible causes are discussed.

Astrophysics