[1] Univ Padua, Dipartimento Fis & Astron Galileo Galilei, I-35131 Padua, Italy
[2] Ist Nazl Fis Nucl, Sez Padova, I-35131 Padua, Italy
[3] Queen Mary Univ London, Sch Phys & Astron, Ctr Res String Theory, London E1 4NS, England
[4] Ecole Normale Super, Phys Theor Lab, F-75231 Paris, France