SIMBAD references

We analyse the standards of the MK system with the help of Hipparcos parallaxes, using only stars for which the error of the absolute magnitude is ≤0.3mag. We find that the main sequence is a wide band and that, although in general giants and dwarfs have different absolute magnitudes, the separation between luminosity classes V and III is not clear. Furthermore, there are a number of exceptions to the strict relation between luminosity class and absolute magnitude. We analyse similarly the system of standards defined by Garrison & Gray (1994AJ....107.1556G) separating low and high rotational velocity standards. We find similar effects as in the original MK system. We propose a revision of the MK standards, to eliminate the most deviant cases.