Based on data obtained at the 1.5-m and 2.1-m telescopes of the Observatorio Astronomico Nacional, San Pedro Mártir, Baja California, México, operated by the Instituto de Astronomía, UNAM.

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Figures 5, 7, 10 and 13 are only available in electronic form at http://www.edpsciences.org

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... IRAF

The IRAF package is written and supported by the IRAF programming group at the National Optical Astronomy Observatories (NOAO) in Tucson, Arizona. NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation (NSF).

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... ADHOCw

The ADHOCw reduction package is available at http://www-obs.cnrs-mrs.fr/ADHOC/

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