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AIMS. Our aims are twofold. First we aim to evaluate the robustness andaccuracy of stellar parameters and detailed elemental abundances that can bederived from high-resolution spectroscopic observations of microlensed dwarfand subgiant stars. We then aim to use microlensed dwarf and subgiant stars toinvestigate the abundance structure and chemical evolution of the Milky WayBulge. [ABRIDGED] METHODS. We present a detailed elemental abundance analysisof OGLE-2008-BLG-209S, the source star of a new microlensing event towards theBulge, for which we obtained a high-resolution spectrum with the MIKEspectrograph on the Magellan Clay telescope. We have performed four differentanalyses of OGLE-2008-BLG-209S. [ABRIDGED] We have also re-analysed threeprevious microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, andOGLE-2007-BLG-349S with the same method. This homogeneous data set, althoughsmall, enables a direct comparison between the different stellar populations.RESULTS. We find that OGLE-2008-BLG-209S is a subgiant star that has ametallicity of [Fe/H] ~-0.33. It possesses [alpha/Fe] enhancements similar towhat is found for Bulge giant stars at the same metallicity, and what also isfound for nearby thick disc stars at the same metallicity. In contrast, theprevious three microlensing dwarf stars have very high metallicities,[Fe/H]>+0.4, and more solar-like abundance ratios, i.e. [alpha/Fe]~0. Thedecrease in the [alpha/Fe] ratio with [Fe/H] is the typical signature ofenrichment from low and intermediate mass stars. We furthermore find that theresults for the four Bulge stars, in combination with results from studies ofgiant stars in the Bulge, seem to favour a secular formation scenario for theBulge.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S