First Author:   Ludovic Renou European University Institute and CODE, Universitat Autonoma de Barcelona ludovic.renou@iue.it   European University Institute, Department of Economics, Villa San Paolo, Via della Piazzuola 43 Firenze, 50133, Italy       http://www.iue.it/Personal/Researchers/renou/ Second Author: Guillaume Carlier Bordeaux I, MAB, UMR CNRS 5466 and Bordeaux IV, GRAPE, UMR CNRS 5113 Guillaume.Carlier@math.u-bordeaux.fr   Universite Bordeaux IV, GRAPE, Avenue Leon Duguit Pessac, 33608, France