Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Heverlee, Belgium
* Physics Department, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
* Physics and Astronomy Department, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussel, Belgium