Based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, program 070.A-9007(A), and on data obtained at the Canada-France-Hawaii Telescope, operated by the CNRS in France, CNRC in Canada and the University of Hawaii.

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Present address: Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127, Bologna, Italy.

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