  In this paper we present the WeCAPP variable star catalogue towards the bulge
of M31. The observations of the WeCAPP microlensing survey (optical R and I
bands) during three years (2000-2003) result in a database with unprecedented
time coverage for an extragalactic variable star study. We detect 23781
variable sources in a 16.1' x 16.6' field centered on the nucleus of M31. The
catalogue of variable stars contains the positions, the periods and the
variations in the R and I bands. We classify the variables according to their
position in the R-band period-amplitude plane. Three groups can be
distinguished; while the first two groups can be mainly associated with
Cepheid-like variables (population I Cepheids in group I, type II Cepheids and
RV Tauri stars in group II), the third one consists of Long Period Variables
(LPVs). We detect 37 RV Tauri stars and 11 RV Tauri candidates which is one of
the largest collections of this class of stars to date. The classification
scheme is supported by Fourier decomposition of the light curves. Our data
shows a correlation of the low-order Fourier coefficients Phi_21 with Phi_31
for classical Cepheids, as well for type II Cepheids and RV Tauri stars.
Correlating our sample of variable stars with the X-ray based catalogues of
Kaaret (2002) and Kong et al. (2002) results in 23 and 31 coincidences, 8 of
which are M31 globular clusters. The number density of detected variables is
clearly not symmetric, which has to be included in the calculations of the
expected microlensing event rate towards M31. This asymmetry is due to the
enhanced extinction in the spiral arms superimposed on the bulge of M31 which
reduces the number of sources to about 60%, if compared to areas of equivalent
bulge brightness (without enhanced extinction being present).
