1. Space Radio-Diagnostics Research Centre, University of Warmia and Mazury in Olsztyn, Prawocheńskiego 9, 10-720 Olsztyn, Poland
2. Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussels, Belgium
3. Faculty of Mathematics and Computer Sciences, University of Warmia and Mazury in Olsztyn, Słoneczna 54, 10-720 Olsztyn, Poland
4. Department of Astronomy and Astrophysics, The University of Manchester, Manchester M13 9PL