  Parallax data from the Hipparcos mission allow the direct distance to open
clusters to be compared with the distance inferred from main sequence (MS)
fitting. There are surprising differences between the two distance
measurements, which could lead to consequences of significant astrophysical
importance. We examine the different possibilities, focusing on MS fitting in
both metallicity-sensitive B-V and metallicity-insensitive V-I for five
well-studied systems (the Hyades, Pleiades, Alpha Per, Praesepe, and Coma Ber).
  The Hipparcos distances to the Hyades and Alpha Per agree with the MS fitting
distances in both colors; there is a possible conflict for Praesepe and Coma
Ber. The Hipparcos distance to the Pleiades disagrees with the MS fitting
distance in both colors at more than the three sigma level. Changes in the
cluster metal abundance, helium abundance, reddening and age-related effects
are all shown to be unlikely to explain the puzzling behavior for the Pleiades.
We present evidence for spatially dependent systematic errors at the 1 mas
level in the parallaxes of Pleiades stars. The implications of this result are
discussed.

Astrophysics