First Author:   Franck Martin University of Rennes 1 and CREM franck.martin@univ-rennes1.fr   Franck Martin, 7, Place Hoche, 35065 Rennes Cedex Rennes, 35065, France   33(0)223233577(Phone) 33(0)223233577(Fax) http://perso.univ-rennes1.fr/franck.martin/ Second Author: Mai lan Nguyen University of Rennes 1 and CREM hlrennes@gmail.com   7, Place Hoche, 7, Place Hoche, 35065 Rennes Cedex Rennes, 35065, France   33(0)651508709(Phone)