1. Department of Physics, Faculty of Science, University of Mauritius , Reduit MU 80837, Mauritius
2. Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy , Alan Turing Building, The University of Manchester, Manchester M13 9PL, UK
3. Leiden Observatory, Leiden University , PO Box 9513, RA Leiden 2300, The Netherlands