Table 1 and Fig. 1 are only available in electronic form at http://www.aanda.org

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Individual photometric measurements are 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/486/505

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