tokens: This work is based on observations collected at the European Southern Observatory, Chile, and the Gran Telescopio de Canarias (GTC), installed in the Spanish Observatory del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. LTSC is supported by the FPI Program by the Ministerio de Economía y Competividad (MINECO) under grant AYA2011-22614. LTSC would like to thank Luis Peralta de Arriba for comments that helped to improve the presentation of the results. JG-R acknowledges support from Severo Ochoa excellence program (SEV-2011-0187) post-doctoral fellowship.
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