1. Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
2. Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
3. Instituto de Telecomunicações, Campus Universitário de Santiago, P-3810-193 Aveiro, Portugal
4. Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, P-3810-193 Aveiro, Portugal
5. Sternberg Astronomical Institute, Moscow State University, Universitetskii pr. 13, 119992 Moscow, Russia