The absolute magnitudes of the G to M type MK standards from the Hipparcosparallaxes

We analyse a sample of about 500 MK standards of cool spectral types (G to M) for to compare the visual absolute magnitudes obtained from both Hipparc os data and Schmidt-Kaler calibrations. Our purpose is to validate our spec troscopic work (Ginestet et al. 1997, 1999) on stars with composite spectra with the help of Hipparcos data. Contrary to what is claimed in other papers, the absolute magnitude domain devoted to the giant stars does not overlap the domain of dwarfs. We find t hat the discrepancies between absolute magnitudes from Hipparcos data and a bsolute magnitudes deduced from Schmidt-Kaler calibrations increase with th e relative error sigma(pi)/pi on the parallaxes. So, for sigma(pi)/pi less than or equal to 0.05 only 3% of the stars present a discrepancy of one lum inosity class, while this percentage reaches 54% for 0.25 < sigma(pi)/pi le ss than or equal to 0.50. Curiously, the luminosity of the giants seems to increase with the distance of the stars, whereas the supergiants of the sample appear underluminous a t least for d < 600 pc! We point out a list of 14 MK standards whose luminosity classes may be erro neous and need a new spectral classification, in the near infrared. The cas e of composite-spectrum binaries is also discussed. Most of these are too d istant for accurate parallaxes ever? with Hipparcos: only sixteen stars hav e sigma(pi)/pi less than or equal to 0.10; for these, we give new spectral classifications in agreement with both our classifications in the near infr ared of the cool components and Hipparcos data. Finally, for stars having high-precision parallaxes (sigma(pi)/pi less than or equal to 5%) there is no serious problem for Schmidt-Kaler calibrations with respect to Hipparcos data. The data corresponding to parallaxes of lo wer precisions should be used with caution and only for statistical analyse s.