1. Institut für Astronomie und Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, D-72076 Tübingen, Germany
2. School of Physics and Astronomy, E.C. Stoner Building, The University of Leeds, Leeds LS2 9JT, UK
3. Institute of Fluid Mechanics and Heat Transfer, TU Wien, Vienna A-1060, Austria
4. Department of Astrophysics, The University of Vienna, Vienna, A-1180, Austria
5. Research Institute of Physics, Southern Federal University, Stachki 194, Rostov-on-Don, 344090, Russia