Thanks Georg and Kevin. It is quite clear now. I was indeed a little bit confused by lstart. Thomas -- Thomas MAZET Laboratoire de Chimie du Solide Minéral (UMR 7555) Université Henri Poincaré-Nancy I BP 239 54506 Vandoeuvre les Nancy Cedex FRANCE Tel.: +33-(0)3-83-68-46-77 Fax : +33-(0)3-83-68-46-11