Context :

Aims : Our aims are twofold .
First we aim to evaluate the robustness and accuracy of stellar parameters and detailed elemental abundances that can be derived from high-resolution spectroscopic observations of microlensed dwarf and subgiant stars .
We then aim to use microlensed dwarf and subgiant stars to investigate the abundance structure and chemical evolution of the Milky Way Bulge .
Contrary to the cool giant stars , with their extremely crowded spectra , the dwarf stars are hotter , their spectra are cleaner , and the elemental abundances of the atmospheres of dwarf and subgiant stars are largely untouched by the internal nuclear processes of the star .

Methods : We present a detailed elemental abundance analysis of OGLE-2008-BLG-209S , the source star of a new microlensing event towards the Bulge , for which we obtained a high-resolution spectrum with the MIKE spectrograph on the Magellan Clay telescope .
We have performed four different analyses of OGLE-2008-BLG-209S .
One method is identical to the one used for a large comparison sample of F and G dwarf stars , mainly thin and thick disc stars , in the Solar neighbourhood .
We have also re-analysed three previous microlensed dwarf stars OGLE-2006-BLG-265S , MOA-2006-BLG-099S , and OGLE-2007-BLG-349S with the same method .
This homogeneous data set , although small , enables a direct comparison between the different stellar populations .

Results : We find that OGLE-2008-BLG-209S is a subgiant star that has a metallicity of [ Fe / H ] \approx - 0.33 .
It possesses [ \alpha /Fe ] enhancements similar to what is found for Bulge giant stars at the same metallicity , and what also is found for nearby thick disc stars at the same metallicity .
In contrast , the previous three microlensing dwarf stars have very high metallicities , [ Fe / H ] \gtrsim + 0.4 , and more solar-like abundance ratios , i.e . [ \alpha / Fe ] \approx 0 .
The decrease in the [ \alpha /Fe ] ratio with [ Fe/H ] is the typical signature of enrichment from low and intermediate mass stars .
We furthermore find that the results for the four Bulge stars , in combination with results from studies of giant stars in the Bulge , seem to favour a secular formation scenario for the Bulge .

Conclusions :