1. Max Planck Institute for Astrophysics , Karl-Schwarzschild-Str 1, D-85741 Garching b. München, Germany
2. Kazan Federal University, Department of Astronomy and Satellite Geodesy , 420008 Kazan, Russia
3. Space Research Institute, Russian Academy of Sciences , Profsoyuznaya 84/32, Moscow 117997, Russia
4. Department of Physics and Astronomy, University of Turku , FI-20014 Turku, Finland