1. Department of Physics and Astronomy, University of Leicester , Leicester LE1 7RH, UK
2. Institute of Astronomy, Russian Academy of Sciences , 48 Pyatnitskaya St., Moscow 119017, Russia
3. Institut für Physik und Astronomie, Universität Potsdam , Karl-Liebknecht-Strasse 24/25, D-14476 Potsdam, Germany
4. University of Vienna, Department of Astrophysics , Vienna 1180, Austria