First Author:   Sacha Bourgeois-gironde University Aix-Marseille 1 - CEPERC & Institut Jean-Nicod sbgironde@gmail.com   Institut Jean-Nicod, Ecole Normale Supérieure, 29 rue dUlm Paris, 75005, France   33-1-44322669(Phone) 33-1-44322669(Fax) Second Author: Anne Corcos University of Picardie - LEM anne.corcos@u-picardie.fr   5/7 avenue Vavin, 5/7 avenue Vavin Paris, 75006, France