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. Ble nding is the close association of a Cepheid with one or more intrinsically luminous stars. High-resolution HST images are compared with our ground-bas ed data, obtained as part of the DIRECT project, for a sample of 22 Cepheid s in the M31 galaxy. The average (median) V-band flux contribution from lum inous companions that 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% lev el for distances. The current Cepheid distance estimates to M31 are all gro und based and are thus affected (underestimated). We discuss indirect metho ds to find which Cepheids are blended, e.g., by the use of well-sampled lig ht curves in at least two optical bands. More generally, our ground-based r esolution in M31 corresponds to the HST resolution at about 10 Mpc. Blendin g leads to systematically low distances in the observed galaxies and theref ore to systematically high estimates of Ho; we discuss the issue and the im plications.