A revised HRD for individual components of binary systems from BaSeL

Johnson BVRI photometric data for individual components of binary systems have been provided by ten Brummelaar et al. (2000). This is essential because non interacting binaries can be considered as two single stars and therefore have to play a critical role in testing and calibrating single-star stellar evolution sets of isochrones and the implicit theory. While they derived the effective temperature (T eff) from their estimated spectral type, we infer metallicity-dependent T eff from a minimizing method fitting the B−V, V−R and V−I colours. For this purpose, a grid of 621,600 flux distributions were computed from the Basel Stellar Library (BaSeL 2.2) of model-atmosphere spectra, and their theoretical colours compared with the observed photometry. As a matter of fact, the BaSeL colours show a very good agreement with the BVRI metallicity-dependent empirical calibrations of Alonso et al. (1996), temperatures being different by 3±3% in the range 4000-8000 K for dwarf stars. Before deriving the metallicity-dependent T eff from the BaSeL models, we paid particular attention to the influence of reddening and stellar rotation. We inferred the reddening from two different methods: (i) the MExcessNg code v1.1 (Méndez & van Altena 1998) and (ii) neutral hydrogen column density data. A comparison of both methods shows a good agreement for the sample which is located inside a local sphere of ∼500 pc, but we point out a few directions where the MExcess model overestimates the E(B−V) colour excess. Influence of stellar rotation on the BVRI colours can be neglected except for 5 stars with large v sin i, the maximum effect on temperature being less than 5%. Our final determinations provide effective temperature estimates for each component. They are in good agreement with previous spectroscopic determinations available for a few primary components, and with ten Brummelaar et al. below ∼10,000 K. Nevertheless, we obtain an increasing disagreement with their temperatures beyond 10,000 K. Finally, we provide a revised Hertzsprung-Russell diagram (HRD) for the systems with the more accurately determined temperatures.