Quasi-consistent filter passbands and the homogeneity of photometric indices

The authors present observational data which clearly illustrate that slight differences in filter passbands cause large systematic effects in the transformation of some photometric indices. The effect is surprisingly large in the case of early B stars where in particular the Strömgren m[SUB]1[/SUB] and c[SUB]1[/SUB] indices are affected. The possible consequences for the position of the β Cephei instability strip in the observational H-R diagram of the young reddened cluster NGC 3293 are discussed.