1. Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
2. Space Research Institute, Austrian Academy of Sciences, A-8042 Graz, Austria
3. Department of Physics, University of Warwick, Coventry CV4 7AL, UK
4. Department of Physics, Columbia University, New York 10027, USA
5. Department of Astronomy, University of California, Berkeley 94270, USA
6. Department of Physics, Indiana University of Pennsylvania, Indiana 15705, USA
7. Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown 06459, USA
8. Department of Physics and Astronomy, Uppsala University, E-752 36 Uppsala, Sweden
9. Department of Astronomy, Columbia University, New York 10027, USA