1. Sub-department of Astrophysics, University of Oxford , DWB, Keble Road, Oxford OX1 3RH, UK
2. Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University , Box 43, SE-221 00 Lund, Sweden
3. Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University , 1800 Sherman Ave, Evanston, IL 60201, USA
4. Department of Physics and Astronomy, University College London , London WC1E 6BT, UK
5. Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg , UMR 7550, F-67000 Strasbourg, France
6. Max-Planck-Institut für Astronomie , Königstuhl 17, D-69117 Heidelberg, Germany
7. Department of Physics, University of Surrey , Guildford GU2 7XH, UK