1. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
2. Clare College, Trinity Lane, Cambridge CB2 1TL, UK
3. Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany
4. Astrophysics Group, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK