The work presented here is based in part on data obtained with the ESO facilities on La Silla (EFOSC/3.6-m).

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Figures 7 and 8 are only available in the electronic form at http://www.edpsciences.org

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...1,

Present address: Gemini Observatory, 670 N. A'ohoku Place, Hilo, HI 96720, USA.

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... IRAF

IRAF is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.

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