tokens: Thanks are due to R. Amorín, Y. Ascasibar, A. B. Morales-Luis, P. Papaderos, and J. Vílchez, who participated in the target selection and in some of the observing time proposals that produced the spectra analyzed in the paper. We also thank an anonymous referee whose comments helped to improve the readability of the paper. This article is based on observations made with the WHT operated by the ING in the Observatorio del Roque de los Muchachos, La Palma, Spain. This work has been partly funded by the Spanish Ministry of Economy and Competitiveness, projects Estallidos AYA2013–47742–C04–02–P and AYA2013–47742–C04–01–P. A.O.-G. thanks Fundación La Caixa for financial support in the form of a PhD contract. M.E.F. is partly funded by the Estallidos project, as well as by the Fundao para a Cincia e Tecnologia (FCTPortugal), through the research grant SFRH/BPD/107801/2015. J.M.-A. acknowledges support from the European Research Council Starting Grant (SEDmorph; P.I. V. Wild).
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