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tokens: We are grateful to the referee for his/her constructive comments. This paper is based on data obtained at the European Southern Observatory, Chile, with proposals ESO 68.C-0149(A), ESO 70.C-0008(A) and ESO 382.C-0195(A). This work has been funded by the Spanish Ministerio de Economía y Competitividad (MINECO) under project AYA2011-22614. JGR acknowledges support from Severo Ochoa excellence program (SEV-2011-0187) postdoctoral fellowship.