1. Scottish Universities Physics Alliance (SUPA), School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, Fife, UK
2. University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, A-1180 Vienna, Austria
3. SUPA, School of Science and Engineering, University of Dundee, Nethergate, Dundee DD1 4HN, UK