1 Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium; e-mail: marting@oma.be 2 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK 3 Instituut voor Sterrenkunde, Universiteit van Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium 4 Astrophysics Dept, CAB (INTA-CSIC), Crta Ajalvir km 4, 28805 Torrejon de Ardoz, Madrid, Spain 5 Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 Amsterdam, The Netherlands 6 School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, Wales CF24 3YB, UK 7 University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Wien, Austria 8 Dept. of Physics and Astronomy, University of Denver, Mail Stop 6900, Denver, CO 80208, USA 9 Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK 10 Dept of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, 10691 Stockholm, Sweden 11 UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK