[1] Observ Paris, Dept Astrophys Relativiste & Cosmol, CNRS, UMR 8629, F-92195 Meudon, France
[2] Inst Hautes Etud Sci, F-91440 Bures Sur Yvette, France
[3] Univ Cambridge, DAMTP, Ctr Math Sci, Cambridge CB3 0WA, England
[4] Charles Univ Prague, Inst Theoret Phys, CR-18000 Prague 8, Czech Republic