SIMBAD references

We derive the absolute visual magnitudes of 14 OB-stars of luminosity class III to V from the parallaxes measured by Hipparcos. The values of M_V can differ by as much as 1.5mag from the M_V-spectral type calibration. Slowly rotating stars, vsini<100km/s, are significantly fainter in M_V than stars with vsini>100km/s of the same spectral type by about 1 magnitude. We discuss this effect and argue that it is due to the influence of rotation on the assignment of spectral types and luminosity class. Slowly rotating stars are also fainter than the standard M_V-spectral type relation. This effect has important consequences on the distance determinations of stars and clusters from M_v and V.