The Optical Gravitational Lensing Experiment. Investigating the Influence of Blending on the Cepheid Distance Scale with Cepheids in the Large Magellanic Cloud

We investigate the influence of blending on the Cepheid distance scale. Blending leads to systematically low distances to galaxies observed with HST, and therefore to systematically high estimates of the Hubble constant H_0. We select a sample of 43 long-period, large-amplitude Cepheids in the Large Magellanic Cloud, from the catalog recently released by the Optical Gravitational Lensing Experiment. We then model the effects of blending, as observed by HST at large distances, by adding the V,I-bands contributions from nearby bright stars. We find that the derived distance modulus would be too short, compared to the true value, by ~0.07 mag at distance of 12.5 Mpc and by \~0.35 mag at distance of 25 Mpc. This has direct and important implications for the Cepheid distances to galaxies observed by the HST Key Project on the Extragalactic Distance Scale and other teams: half of the KP sample is likely to exhibit a blending bias greater than 0.1 mag.