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We have investigated how the K-band magnitude of the red clump $[M_K(RC)]$depends on age and metallicity, using 2MASS infrared data for a sample of 24 open clusters with known distances. We show that a constant value of $M_K(RC)=-1.57 \pm 0.05$is a reasonable assumption to use in distance determinations for clusters with metallicity between -0.5 and +0.4 dex and age between 108.5and 109.9years.

Context. Recent progress in the seismic interpretation of field βCep 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 βCep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known βCep 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 NGC 884. Fifteen different instruments collected almost 77 500 CCD images in 1286 h. 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 Iand 5.3 mmag in U. The noise level in the amplitude spectra is 50 μmag in the Vband. 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.

Context. Recent progress in the seismic interpretation of field βCep 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.