1. Institute for Condensed Matter Physics, Svientsitskii St. 1, 79011 Lviv, Ukraine
2. Institute of Physics, University of Zielona Góra, Licealna St. 9, 65-417 Zielona Góra, Poland
3. Janusz Gil Institute of Astronomy, University of Zielona Góra, Licealna St. 9, 65-417 Zielona Góra, Poland