  CONTEXT: Recent progress in the seismic interpretation of field beta Cep
stars has resulted in improvements of the physics in the stellar structure and
evolution models of massive stars. Further asteroseismic constraints can be
obtained from studying ensembles of stars in a young open cluster, which all
have similar age, distance and chemical composition.
  AIMS: To improve our comprehension of the beta Cep stars, we studied the
young open cluster NGC 884 to discover new B-type pulsators, besides the two
known beta Cep stars, and other variable stars.
  METHODS: An extensive multi-site campaign was set up to gather accurate CCD
photometry 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 into
interpretable differential light curves. Various variability indicators and
frequency analyses were applied to detect variable stars in the field. Absolute
photometry 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 the
amplitude spectra is 50 micromag in the V band. Our campaign confirms the
previously known pulsators, and we report more than one hundred new multi- and
mono-periodic B-, A- and F-type stars. Their interpretation in terms of
classical instability domains is not straightforward, pointing to imperfections
in theoretical instability computations. In addition, we have discovered six
new eclipsing binaries and four candidates as well as other irregular variable
stars in the observed field.
