1. Leiden Observatory, Leiden University, Neils Bohrweg, 2333 CA Leiden, Netherlands
2. Center for Theoretical Physics, Polish Academy of Sciences, al. Lotników 32/46, PL-02-668 Warsaw, Poland
3. Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
4. Monash Centre for Astrophysics, Monash University, Clayton, VIC 3800, Australia
5. Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany