1. South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
2. Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa
3. National Astrophysics and Space Science Programme, Department of Mathematics and Applied Mathematics, University of Cape Town, 7701 Rondebosch, South Africa
4. Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium