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D. Bersier (Harvard-Smithsonian Center for Astrophysics)
We analyze archival HST data of the galaxy NGC 4603. This galaxy has been observed to discover Cepheids and derive a distance (Newman et al. 1999). It is the most distant galaxy in which Cepheids have been found. However it is possible that the distance is affected by systematic errors, most particularly the fact that non-variable stars contaminate the sample of Cepheids. We use the method known as image subtraction (ISIS) to search for Cepheids. It is now clear that this technique is a much more efficient way to search for variable stars in very crowded fields than standard photometric methods. However this case is particularly challenging since WFPC2 seriously undersamples the PSF. We discuss the procedure adopted to deal with undersampled images. We compare the ISIS light curves of candidate Cepheids with those obtained with standard PSF-fitting photometry and we assess the reality of the variables found previously. We discuss the impact on the distance and the prospects for finding variable stars at large distances (30 Mpc).