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CONTEXT: Recent progress in the seismic interpretation of field beta Cepstars has resulted in improvements of the physics in the stellar structure andevolution models of massive stars. Further asteroseismic constraints can beobtained from studying ensembles of stars in a young open cluster, which allhave similar age, distance and chemical composition.  AIMS: To improve our comprehension of the beta Cep stars, we studied theyoung open cluster NGC 884 to discover new B-type pulsators, besides the twoknown beta Cep stars, and other variable stars.  METHODS: An extensive multi-site campaign was set up to gather accurate CCDphotometry time series in four filters (U, B, V, I) of a field of NGC884.Fifteen different instruments collected almost 77500 CCD images in 1286 hours.The images were calibrated and reduced to transform the CCD frames intointerpretable differential light curves. Various variability indicators andfrequency analyses were applied to detect variable stars in the field. Absolutephotometry was taken to deduce some general cluster and stellar properties.  RESULTS: We achieved an accuracy for the brightest stars of 5.7 mmag in V,6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in theamplitude spectra is 50 micromag in the V band. Our campaign confirms thepreviously known pulsators, and we report more than one hundred new multi- andmono-periodic B-, A- and F-type stars. Their interpretation in terms ofclassical instability domains is not straightforward, pointing to imperfectionsin theoretical instability computations. In addition, we have discovered sixnew eclipsing binaries and four candidates as well as other irregular variablestars in the observed field.

Photometric multi-site campaign on the open cluster NGC 884 I. Detection of the variable stars