The Araucaria Project: The Effect of Blending on the Cepheid Distance to NGC 300 from Advanced Camera for Surveys Images1

We have used the Advanced Camera for Surveys aboard the Hubble Space Telescope (HST) to obtain F435W, F555W, and F814W single-epoch images of six fields in the spiral galaxy NGC 300. Taking advantage of the superb spatial resolution of these images, we have tested the effect that blending of the Cepheid variables studied from the ground with close stellar neighbors, unresolved on the ground-based images, has on the distance determination to NGC 300. Out of the 16 Cepheids included in this study, only three are significantly affected by nearby stellar objects. After correcting the groundbased magnitudes for the contribution by these projected companions to the observed flux, we find that the corresponding period-luminosity relations in V, I, and the Wesenheit magnitude WI are not significantly different from the relations obtained without corrections. We fix an upper limit of 0.04 mag to the systematic effect of blending on the distance modulus to NGC 300. As part of our HST imaging program, we present improved photometry for 40 blue supergiants in NGC 300.