1. School of Mathematics and Physics, The University of Queensland, St. Lucia, QLD 4072, Australia
2. European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
3. INAF Osservatorio Astronomico di Bologna, via Gobetti 93/3, I-Bologna 40129, Italy
4. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA