Based on observations with the P7 photometer at the 0.7 m Swiss Telescope, La Silla (Chile) and at the Flemish Mercator Telescope, La Palma (Spain) and on observations with the Submillimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), Mauna Kea, Hawaii.

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Appendices A-C are only available in electronic form at http://www.edpsciences.org

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