First Author:   Frédérique Bec THEMA, University of Cergy-Pontoise and CREST, France bec@ensae.fr   CREST-LMA, Timbre J360, 15, Bd Gabriel Péri MALAKOFF, 92245, France   +33 1 41 17 77 82(Phone) +33 1 41 17 77 82(Fax) Second Author: Anders Rahbek Dept. of Economics, Copenhagen, Denmark Anders.Rahbek@econ.ku.dk         Third Author: Mélika Ben Salem OEP, Paris-Est University and LEA-INRA (PSE), France mbs@univ-mlv.fr