REDDENING EFFECTS IN STROMGREN PHOTOMETRY

We analyse the standard indices of B-type stars in uvby photometry to investigate whether and to what extent these indices could be affected by systematic trends caused by non-negligible reddening. To perform the analysis a sample of 17 young open clusters with uvby published photometric studies, carried out by various authors and using different instrumental equipments, has been selected. The sample only includes clusters having B-type members with published MK spectral types. The results show that the standard values of the c1 index indeed contain systematic errors, which are larger for larger reddening, and can be expressed as a linear dependence of the observed reddening slope E(c1)MK/E(b-y)MK on spectral type. We suggest a simple explanation to account for this observational evidence, based on reddening dependent coefficients in the transformation from instrumental to standard quantities in the Stromgren system. These kind of tendencies are not so marked for the m1 and (b-y) standard values as for the c1 index. The main trends shown by the m1 and (b-y) sequences could be associated with intrinsic peculiarities of reddening in some of the clusters included in the sample.