[1] Argelander-Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
[2] Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
[3] Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, United States