Based in part on observations collected at the Observatoire de Haute-Provence (OHP).

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Photometry data mentioned in Sect. 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/441/451

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