1. Department of Astronomy, Saint Petersburg State University , Saint-Petersburg 198504, Russia
2. Space Research Institute of the Russian Academy of Sciences , Profsoyuznaya Str. 84/32, Moscow 117997, Russia
3. Department of Physics and Astronomy, University of Turku , FI-20014 Turku, Finland
4. Astrophysics, Department of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
5. Leiden Observatory, Leiden University , NL-2300 RA Leiden, The Netherlands