Based on observations collected at ESO VLT-UT2 under programme 72.D-0507.

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Complete Tables 2, 3 and 5 are only available in electronic form at http://www.edpsciences.org

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... $\rm [O/Fe]\sim -1$ dex

We use the usual spectroscopic notation: log n(A) is the abundance (by number) of element A in the usual scale where log n(H) = 12; notation [A/H] is the logarithmic ratio of the abundances of elements A and H in the star, minus the same quantity in the Sun.

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... mode

The analysis of these higher resolution spectra will be presented elsewhere.

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... spectra

This value is slightly different from what we derive in the present study; however, the dependence of (V-K) on [Fe/H] is so weak that temperatures are almost unaffected by this difference.

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... IRAF

IRAF is distributed by the National Optical Astronomical Observatory, operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.

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... widths

EWs are available upon request from the first author.

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