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 ( \sim 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 ( \sim 0.1 mag ) .
Since large amplitude Cepheids are less affected by blending , we have used those with an amplitude \mathcal { A } _ { V } > 0.8 mag to derive a distance to M 31 of ( m - M ) _ { 0 } = 24.32 \pm 0.12 mag .