[1] Space Telescope Sci Inst, Baltimore, MD 21218 USA
[2] ESA, Space Telescope Operat Div, F-75738 Paris 15, France
[3] INAF, Osservatorio Astron Bologna, I-40127 Bologna, Italy
[4] NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
[5] Univ Wisconsin, Dept Astron, Madison, WI 53706 USA
[6] Univ Michigan, Dept Astron, Ann Arbor, MI 48109 USA
[7] Max Planck Inst Astron, D-69117 Heidelberg, Germany
[8] UCL, Dept Phys & Astron, London WC1E 6BT, England
[9] New Mexico State Univ, Dept Astron, Las Cruces, NM 88003 USA