Absolute magnitude calibration of Geneva photometry based on Hipparcos parallaxes: The B-type stars

The Hipparcos data give us access, for the first time, to trigonometric parallaxes of B-type stars. This allows us to refine and re-adjust extant empirical absolute magnitude calibrations of the Geneva X and Y reddening-free photometric parameters on the basis of the parallaxes that are presently available. The new calibration presented here, which is totally independent of other data, is compared with results that were derived with earlier versions. Its validity regarding a selection of Ap stars is tested on the basis of their Hipparcos parallaxes. The calibration must, however, not be considered to be optimal until the whole Hipparcos and Tycho data are distributed and enable a definitive study to be undertaken.