1. School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK.
2. National Radio Astronomy Observatory, 10003 Lopezville Road, Socorrow, NM 87801, USA.
3. American Association of Variable Star Observers, 49 Bay State Road, Cambridge, MA 02138, USA.
4. University of Manchester, Jodrell Bank Observatory, Macclesfield SK11 9DL, UK.