1. Space Research Institute (IKI) , Profsoyuznaya 84/32, 117997 Moscow, Russia
2. Max Planck Institute for Astrophysics , Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany
3. Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München , Scheinerstr.1, D-81679 München, Germany
4. Astro Space Center, P. N. Lebedev Physical Institute of RAS , Profsojuznaya 84/32, 117997 Moscow, Russia