University of Wales at Aberystwyth, Institute of Mathematical and Physical Sciences, Aberystwyth, SY23 3BZ United Kingdom; Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA MA 02138, USA
Bridgewater State College, Department of Physics, Bridgewater, MA MA 02325, USA
Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 238-725, Pasadena, CA CA 91109, USA