WeCAPP - Wendelstein Calar Alto pixellensing project I - Tracing dark and bright matter in M31

We present WeCAPP, a long term monitoring project searching for microlensin g events towards M31. Since 1997 the bulge of M31 was monitored in two diff erent wavebands with the Wendelstein 0.8 m telescope. In 1999 we extended o ur observations to the Calar Alto 1.23 m telescope. Observing simultaneousl y at these two sites we obtained a time coverage of 53% during the observab ility of M31. To check thousands of frames for variability of unresolved so urces, we used the optimal image subtraction method (OIS) by Alard & Lupton (1998). This enabled us to minimize the residuals in the difference image analysis (DIA) and to detect variable sources with amplitudes at the photon noise level. Thus we can detect microlensing events with corresponding amp lifications A >10 of red clump giants with M-I = 0.