  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 (M31), is crucial to obtaining
reliable distance determinations. We obtained accurate photometry (in B and V
passbands) of 416 Cepheids in M31 over a five year campaign within a survey
aimed at the detection of eclipsing binaries. The resulting Cepheid sample is
the most complete in M31 and has almost the same period distribution as the
David Dunlap Observatory sample in the Milky Way. The large number of epochs
(~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
M31 (~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 M31 of
(m-M)_0=24.32+/-0.12 mag.
