1. Dunlap Institute for Astronomy and Astrophysics and Department of Astronomy and Astrophysics University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
2. King's College London, Strand, London, WC2R 2LS, UK
3. Institut für Astrophysik, Georg-August Universitat, D-37077 Göttingen, Germany
4. Department of Physics and Astronomy, Haverford College, 370 Lancaster Avenue, Haverford, PA 19041, USA