1. Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
2. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands
3. Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8W 2Y2, Canada
4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA