  We investigate the influence of blending on the Cepheid distance scale.
Blending is the close association of a Cepheid with one or more intrinsically
luminous stars. High-resolution HST images are compared to our ground-based
data, obtained as part of the DIRECT project, for a sample of 22 Cepheids in
the M31 galaxy. The average (median) V-band flux contribution from luminous
companions which are not resolved on the ground-based images is about 19% (12%)
of the flux of the Cepheid. This is a large effect - at the 10% level for
distances. The current Cepheid distance estimates to M31 are all ground-based,
and are thus affected (underestimated). We discuss indirect methods to find
which Cepheids are blended, e.g. by the use of well-sampled light curves in at
least two optical bands. More generally, our ground-based resolution in M31
corresponds to the HST resolution at about 10 Mpc. Blending leads to
systematically low distances in the observed galaxies, and therefore to
systematically high estimates of H_0; we discuss the issue and the
implications.

Astrophysics