1. Astrophysics Group, School of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
2. Institute of Physics and Astronomy, University of Potsdam, D-14476, Potsdam, Germany
3. Research Institute of Physics, Southern Federal University, Stachki 194, Rostov-on-Don, 344090, Russia
4. Department of Astrophysics, The University of Vienna, Vienna, A-1180, Austria
5. Observatoire de Genéve, Université de Genéve, chemin des Maillettes 51, CH-1290 Sauverny, Switzerland
6. Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany