1. Hamburg Observatory, University of Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
2. School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
3. Institute of Experimental Physics, University of Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
4. Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK