1. Kavli Institute for Astronomy and Astrophysics (KIAA), Peking University, 100871 Beijing, China
2. Max–Planck–Institut für Astrophysik (MPA), Karl–Schwarzschild–Str. 1, D-85741 Garching, Germany
3. Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow 117997, Russia
4. Tartu Observatory, Tõravere 61602, Estonia