Observations collected both at the European Southern Observatory, Paranal and La Silla, Chile (ESO programme 65.L-0340) and with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by AURA Inc. under contract to NASA.

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Tables 1, 2, 6 and Fig. 1 are only available in electronic form at http://www.edpsciences.org

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