  In the first of a new series of papers on open cluster distances, we use
updated stellar evolution models to construct an isochrone appropriate for the
Hyades, and compare it with the Hyades eclipsing binary system vB 22. We find
that the absolute and relative luminosities of the two stars are in good
agreement with the model, but the radii do not match the values inferred from
eclipse data. We present evidence that there is a consistency problem with the
flux ratios and the inferred radii, and discuss possible theoretical effects
that could be responsible for the mismatch in the radii. We derive a helium
abundance for the Hyades of Y = 0.271 \pm 0.006, which is equal within the
errors to the Sun's initial helium abundance even though the Hyades is
considerably more metal-rich.

Astrophysics