Pulsating stars in NGC 6231: Frequency analysis and photometric mode identification near the main-sequence

Aims. We used Johnson UBV photometric CCD observations to identify pulsating and other variable stars in the young open cluster NGC 6231. The multi-colour information was used to classify pulsating variables, perform frequency analysis, and, where possible, to compare observed to theoretical amplitude ratios for mode identification. Methods. The CCD data were used to investigate a total of 473 stars in the field. The data reduction was performed with standard IRAF tools, while the extraction of light curves was done with combined aperture and point-spread-function photometry routines delivered with the DAOPHOT package. Differential light curves were obtained by identifying a set of suitable comparison stars and the frequency analysis was then conducted on the basis of Fourier methods. Our classification of pulsating stars was based on the time scales and amplitudes of the variability with respect to the different filters and stellar parameters as calculated from published Strömgren and Geneva photometry. Attempts to set constraints on the pulsation mode were performed for stars with sufficiently high amplitude based on the significant dependence of amplitudes on wavelength. Results. We identified 32 variable stars in the field of the cluster, of which 21 are confirmed members of NGC 6231 and 12 are newly detected variable stars. Ten stars were classified as slowly pulsating B (SPB) stars in NGC 6231, of which seven are new discoveries. We also analysed six previously reported β Cephei variables in more detail. One of them may be a hybrid β Cephei/SPB pulsator. In addition, we also investigated five previously suspected pulsators of this group that we cannot convincingly confirm owing to their small pulsation amplitudes. The remaining 11 variable stars are either not members of NGC 6231 or the membership status is questionable. Among them are three previously known δ Scuti stars, two newly detected pulsators of this class, one new and two already known eclipsing binaries, one new SPB variable, one possible pre-main-sequence pulsator, and another new variable star for which we cannot present a classification. With more than 20 main-sequence pulsators of spectral type B, NGC 6231 becomes the open cluster with the largest population of such pulsating stars known.