[1] European Space Agcy, Inst Astrophys, Paris, France
[2] Univ Bologna, I-40126 Bologna, Italy
[3] Inst Astrophys, F-75014 Paris, France
[4] Acad Sinica, Taipei, Taiwan
[5] CALTECH, Pasadena, CA 91125 USA
[6] Observ Paris, Meudon, France
[7] CALTECH, Pasadena, CA 91125 USA