Concerning the Slope of the Cepheid Period-Luminosity Relation
  We discuss the impact of possible differences in the slope of the Cepheid
Period-Luminosity relation on the determination of extragalactic distances in
the context of recent studies that suggest changes in this slope. We show that
the Wesenheit function W = V - R x ((V-I), widely used for the determination of
Cepheid distances, is expected to be highly insensitive to changes in the slope
of the underlying (monochromatic) Period-Luminosity (PL) relations. This occurs
because the reddening trajectories in the color-magnitude plane are closely
parallel to lines of constant period. As a result W-based Period-Luminosity
relations have extremely low residual dispersion, which is because differential
(and total line-of-sight) reddening is eliminated in the definition of W and
the residual scatter due to a star's intrinsic color/position within the
Cepheid is also largely insensitive to W. Basic equations are presented and
graphically illustrated, showing the insensitivity of W to changes in the
monochromatic PL relations.
