  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 102 Cepheids in
the M33 galaxy. The average (median) flux contribution from luminous companions
not resolved on the ground-based images in the B, V and I bands (S_B, S_V, S_I)
is about 29% (15%), 24% (14%), 30% (21%) of the flux of the Cepheid. For 64
Cepheids with periods in excess of 10 days the average (median) values of S_B,
S_V, S_I are 20% (10%), 16% (7%), 23% (14%). This shows that, depending on the
sample of objects chosen, the distance derived from our ground-based V-band
magnitudes for the M33 Cepheids would be systematically underestimated by about
8%-11% (3%-6%). Using artificial star tests we study crowding and blending as
separate phenomena. These tests indicate that the effect of blending could be
more significant than crowding in regions of lower surface brightness, below
21.6 mag/arcsec^2. We also find indications that in the case of long period
Cepheids (P>10^d) companions from blending are on average bluer than companions
introduced by crowding. Our ground-based resolution in M33 corresponds to the
HST resolution at about 11 Mpc. We estimate the effect of blending at
resolutions corresponding to a range of distances as observed with the HST
using the HST M33 data as the template unblended population. We compare our
results to those obtained by other groups.
