1. European Southern Observatory, Karl-Schwarzschild-str 2, D-85748 Garching bei München, Germany
2. DAMTP, Centre for Mathematical Sciences, University of Cambridge, Cambridge CB3 0WA, UK
3. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
4. School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
5. INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34143 Trieste, Italy