[1] Space Telescope Sci Inst, Baltimore, MD 21218 USA
[2] Astron Observ Padova, I-351223 Padua, Italy
[3] Penn State Univ, Dept Astron, University Pk, PA 16802 USA
[4] Univ Insubria, I-22100 Como, Italy
[5] Univ Melbourne, Sch Phys, Parkville, Vic 3052, Australia