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D. Sasselov, M. Krockenberger, K. Z. Stanek, J. Kaluzny, J. P. Beaulieu, J. Tonry, M. Mateo (CfA)
This is a first step in the ongoing program to improve the direct distance estimate to two important galaxies in the cosmological distance ladder -- M31 and M33. The massive photometry we have obtained as part of our project DIRECT over the past 3 years provides us with very good light curves for known and new Cepheid variables. The light curves afford meaningful Fourier decompositions. As a result we are able to study the signature of the resonance center between the fundamental and the second overtone pulsational modes. Resonance signatures are sensitive to metallicity as already established by Cepheid studies in LMC, SMC, and the Galaxy. We make use of them to distinguish between Cepheids of different populations and metallicity inside each galaxy (M31 and M33). The handle on metallicity and large number of new Cepheids with very good BVI photometry helps us build improved period-luminosity (PL) relations for each galaxy. We derive their distances by using the standard technique of comparing PL relations, but are able to correct individually for extinction. The ultimate goal of project DIRECT is to derive direct Baade-Wesselink distances to Cepheids in M31 and M33, as well as direct distances to detached eclipsing binaries.