1. Department of Physics and Astronomy, University of the Western Cape , Cape Town, 7535, South Africa
2. School of Earth and Space Exploration, Arizona State University , Tempe, AZ 85287-6004, USA
3. Kapteyn Astronomical Institute, University of Groningen , PO Box 800, NL-9700AV Groningen, the Netherlands
4. Physics Department, Winona State University , Pasteur 120, Winona, MN 55987, USA