The Chemical Composition of Two Supergiants in the Dwarf Irregular Galaxy WLM

[Abridged] The chemical composition of two stars in WLM has been determined from high quality UVES data obtained at the VLT UT2 (program 65.N-0375). The model atmospheres analysis shows that they have the same metallicity, [Fe/H] = -0.38 +/-0.20, and [Mg/Fe] = -0.24 +/-0.16. This result suggests that the [alpha(Mg)/Fe] ratio in WLM may be suppressed relative to solar abundances (also supported by differential abundances relative to similar stars in NGC6822 and the SMC). The absolute Mg abundance, [Mg/H] = -0.62 is high relative to what is expected from the nebulae though, where two independent spectroscopic analyses of the HII regions in WLM yield [O/H] = -0.89. Intriguingly, the oxygen abundance determined from the OI 6158 feature in one WLM star is [O/H] = -0.21 +/-0.10, corresponding to five times higher than the nebular oxygen abundance. This is the first time that a significant difference between young stellar and nebular oxygen abundances has been found, and presently, there is no simple explanation for this difference. If the stellar abundances reflect the true composition of WLM, then this galaxy lies well above the metallicity-luminosity relationship for dwarf irregular galaxies. It also suggests that WLM is more chemically evolved than currently interpreted from its color-magnitude diagram.