Metallicity effects in the spectral classification of O-type stars.
  Theoretical consideration
  Based on an exteded grid of NLTE, line blanketed model atmospheres with
stellar winds as calculated by means of FASTWIND, we have investigated the
change in the strengths of strategic Helium transitions in the optical as
caused by a 0.3 decrease in metallicity with respect to solar abundances. Our
calculations predict that only part of the observed increase in Teff, of O-type
dwarfs could be explained by metallicity effects on the spectral type
indicators, while the rest must be attributed to other reasons (e.g., different
stellar structures as a function of metallicity or differences between observed
and theoretical wind parameters etc.). In addition, we found that using the He
II 4686 line to classify stars in low metallicity environments (Z < 0.3 solar)
might artificially increase the number of low luminosity (dwarfs and giants)
O-stars, on the expense of the number of O-supergiants.
