1. Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, E-08860 Castelldefels, Spain
2. Institute for Space Studies of Catalonia, c/Gran Capità 2–4, Edif. Nexus 201, E-08034 Barcelona, Spain
3. Anton Pannekoek Institute for Astronomy, University of Amsterdam, NL-1090 GE Amsterdam, the Netherlands
4. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA, Leiden, the Netherlands