[1] Univ Hertfordshire, STRI, Ctr Astrophys Res, Hatfield AL10 9AB, Herts, England
[2] Rochester Inst Technol, Dept Phys, Rochester, NY 14623 USA
[3] INAF, Osservatorio Astrofis Arcetri, I-50125 Florence, Italy
[4] Univ Oxford, Oxford OX1 3NP, England
[5] INAF, Osservatorio Astron Torino, I-10025 Pino Torinese, Italy
[6] Univ Nottingham, Sch Phys & Astron, Nottingham NG7 2RD, England
[7] ETH, Honggerberg HPF G4 3, CH-8092 Zurich, Switzerland
[8] Space Telescope Sci Inst, Baltimore, MD 21218 USA