[1] UCL, Dept Phys & Astron, London WC1E 6BT, England
[2] Leiden Univ, Leiden Observ, NL-2300 RA Leiden, Netherlands
[3] Univ Coll London, Mullard Space Sci Lab, Dorking RH5 6NT, Surrey, England
[4] Carnegie Mellon Univ, Dept Phys, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA
[5] Heidelberg Univ, Astron Recheninst, Zentrum Astron, D-69120 Heidelberg, Germany
[6] CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA
[7] EPFL, Observ Sauverny, Lab Astrophys LASTRO, CH-1290 Versoix, Switzerland