A comprehensive study of Cepheid variables in the Andromeda galaxy - Period distribution, blending, and distance determination

Extragalactic Cepheids are the basic rungs of the cosmic distance scale. They are excellent standard candles, although their luminosities and corresponding distance estimates can be affected by the particular properties of the host galaxy. Therefore, the accurate analysis of the Cepheid population in other galaxies, and notably in the Andromeda galaxy (M 31),is crucial to obtaining reliable distance determinations. We obtained accurate photometry (in B and V passbands) of 416 Cepheids in M 31 over a five year campaign within a survey aimed at the detection of eclipsing binaries. The resulting Cepheid sample is the most complete in M 31 and has almost the same period distribution as the David Dunlap Observatory sample in the Milky Way. The large number of epochs (similar to 250 per filter) has permitted the characterisation of the pulsation modes of 356 Cepheids, with 281 of them pulsating in the fundamental mode and 75 in the first overtone. The period-luminosity relationship of the fundamental mode Cepheids has been studied and a new approach has been used to estimate the effect of blending. We find that the blending contribution is as important as the metallicity correction when computing Cepheid distance determinations to M 31 (similar to 0.1 mag). Since large amplitude Cepheids are less affected by blending, we have used those with an amplitude A(V) > 0.8 mag to derive a distance to M 31 of (m - M)(0) = 24.32 +/- 0.12 mag.