1. Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences , Pulkovskoye Chaussee 65/1, St Petersburg 196140 , Russia
2. St.Petersburg State University , 7/9 Universitetskaya nab., St.Petersburg 199034 , Russia
3. Department of Physics and Astronomy , N283 ESC, Brigham Young University, Provo, UT 84602 , USA
4. Special Astrophysical Observatory, Russian Academy of Sciences , 369167 Nizhnii Arkhyz , Russia