The DIRECT Project: Influence of Blending on the Cepheid Distance Scale. I. Cepheids in M31

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.