1. Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India
2. Bogolyubov Institute for Theoretical Physics, 14-b Metrologicheskaya St., Kiev 03680, Ukraine
3. Department of Physics, Taras Shevchenko National University of Kiev, 64/13 Vladimirskaya St., Kiev 01601, Ukraine