1. International Centre for Radio Astronomy Research, The University of Western Australia, Crawley, WA, 6009 Australia,
2. Helmholtz-Institut für Strahlen- und Kernphysik (HISKP), Universität Bonn, Nussallee 14-16, D-53115 Bonn, Germany
3. Charles University in Prague, Faculty of Mathematics and Physics, Astronomical Institute, V Holeĺovickách 2, CZ-180 00 Praha 8, Czech Republic
4. European Southern Observatory, Karl-Schwarzschild-StraSSe 2, D-85748 Garching bei München, Germany