We derived an intrinsic colour calibration involving the broad band colours (B-V) and (U-B) and the line indices HP2 and KP. Within its validity range the calibration may be used to derive the reddening E(B-V) of individual stars with an accuracy of the order of 0.03 mag. We showed that the reddenings derived from our calibration are in good agreement with those derived from the Schuster & Nissen (1989) calibration of Strömgren photometry. When compared to reddening determinations based on maps, either those of Burstein & Heiles (1982) or those of Schlegel et al. (1998), our reddenings are systematically higher by about 0.01 mag.
The data base for HP2, KP, (B-V) and (U-B)for HK survey stars is steadily increasing thanks to the efforts of Beers and collaborators and presently data for about 3000 stars are available. Our calibration may be used to derive reddenings for all these stars. Dereddened colours are necessary to determine the metallicity from the Beers et al. (1999) calibration and to derive a photometric distance. In the case of stars which will be observed spectroscopically at high resolution, dereddened colours will allow the determination of effective temperatures from colour-temperature calibrations.
Whenever photometry is available the Schuster & Nissen calibration is the preferred way to determine the reddening of a star. However, when no Strömgren photometry is available, our calibration provides a useful way to estimate reddening with comparable accuracy.
We are grateful to Dr. A. Alonso, for carefully reviewing our paper and for suggesting many additions and improvements to its first version. This research made use of the SIMBAD data base operated at CDS, Strasbourg.
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