Repositorio Institucional de la Universidad de Oviedo

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The authors would like to thank A. Anabal´on for interesting conversations. This work has been supported in part by the Spanish Ministry of Science and Education grants FPA2012- 35043-C02 (-01 & -02), the Centro de Excelencia Severo Ochoa Program grant SEV-2012- 0249, the EU-COST Action MP1210 “The String Theory Universe”, the Principado de Asturias grant GRUPIN14-108 and the Spanish Consolider-Ingenio 2010 program CPAN CSD2007-00042. The work was further supported by the Severo Ochoa pre-doctoral grant SVP-2013-067903 (PF-R). TO wishes to thank M.M. Fern´andez for her permanent support.