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R. Andr6 and J. Villain, Laboratoire de Physique et Chimie de l'environnement

, 45071 Orl6ans Cedex 2, France. (email: raandre@cnrs-orleans.fr; jvillain@cnrs-orlean.fr), Recherche Scientifique

L. Barthes and J. Cerisier,

. Avenue-de-nepture, 94107 Saint-Maur-des-Foss6s Cedex, France. (e-mail: laurent.barthes@ cetp.ipsl.fr; jeanclaude, cerisier?cetp.ipsl.fr)