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Based on an exteded grid of NLTE, line blanketed model atmospheres withstellar winds as calculated by means of FASTWIND, we have investigated thechange in the strengths of strategic Helium transitions in the optical ascaused by a 0.3 decrease in metallicity with respect to solar abundances. Ourcalculations predict that only part of the observed increase in Teff, of O-typedwarfs could be explained by metallicity effects on the spectral typeindicators, while the rest must be attributed to other reasons (e.g., differentstellar structures as a function of metallicity or differences between observedand theoretical wind parameters etc.). In addition, we found that using the HeII 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.

Metallicity effects in the spectral classification of O-type stars. Theoretical consideration