Detection of beat Cepheids in M33 and their use as a probe of the M33 metallicity distribution

Our analysis of the deep CFHT M33 variability survey database has uncovered five beat Cepheids that are pulsating in the fundamental and first-overtone modes. With only the help of stellar pulsation theory and of massluminosity relations, derived from evolutionary tracks, we can accurately determine the metallicities Z of these stars. The [O/H] metallicity gradient of -0.16 dex kpc(-1) that is inferred from the M33 galactocentric distances of these Cepheids and from their "pulsation" metallicities is in excellent agreement with the standard spectroscopic metallicity gradients that are determined from H ii regions, early-B supergiant stars, and planetary nebulae. Beat Cepheids can thus provide an additional, independent probe of galactic metallicity distributions.