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We present, for the first time, a detailed abundance analysis of five giant stars in the moderately metal-poor bulge globular cluster NGC 6558. Spectra have been obtained at the VLT with the multifiber spectrograph FLAMES in GIRAFFE mode (R ~ 22,000). The resulting iron abundance is [Fe/H] = -0.97 ± 0.15, in good agreement with the metallicity inferred from the slope of the red giant branch, but unusually high for a cluster with such a blue horizontal branch, possibly indicating an old age. A color-magnitude diagram in V and I, based on photometry obtained with the Wide-Field Imager at ESO, is also presented. An isochrone of 14 Gyr fits the evolutionary sequences, confirming an old age. NGC 6558 is another "second-parameter" bulge cluster. The metallicity derived is near the end of the low-metallicity tail of the bulge field-star distribution; hence, it presumably formed at the very early stages of the bulge formation. Abundance ratios show enhancements of the α-elements oxygen, magnesium, and silicon, with [O/Fe] = +0.38, [Mg/Fe] = +0.24, [Si/Fe] = +0.23, and solar calcium and titanium. The r-element europium is also enhanced by [Eu/Fe] = +0.36. The odd-Z elements sodium and aluminum, as well as the s-elements barium and lanthanum, show solar ratios. A heliocentric radial velocity of v = -197.3 ± 4 km s-1 is found for NGC 6558.

NGC 6558: A Blue Horizontal Branch Moderately Metal-Poor Globular Cluster in the Bulge