1997A&AS..123...83J

In recent years, several authors have revised the calibrations used to compute physical parameters (M_v, T_eff, log g, [Fe/H]) from intrinsic colours in the uvbyH_β photometric system. For reddened stars, these intrinsic colours can be computed through the standard relations among colour indices for each of the regions defined by Stroemgren (1966ARA&A...4..433S) on the HR diagram. We present a discussion of the coherence of these calibrations for main-sequence stars. Stars from open clusters are used to carry out this analysis. Assuming that individual reddening values and distances should be similar for all the members of a given open cluster, systematic differences among the calibrations used in each of the photometric regions might arise when comparing mean reddening values and distances for the members of each region. To classify the stars into Stroemgren's regions we extended the algorithm presented by Figueras et al. (1991A&AS...87..319F) to a wider range of spectral types and luminosity classes. The observational ZAMS are compared with the theoretical ZAMS from stellar evolutionary models, in the range 6500-30000K. The discrepancies are also discussed.