[1] UNIV PITTSBURGH, DEPT PHYS, PITTSBURGH, PA 15260 USA
[2] INST ASTRON, CAMBRIDGE CB3 0HA, ENGLAND
[3] UCL, LONDON WC1E 6BT, ENGLAND
[4] UNIV CALIF SAN DIEGO, DEPT PHYS, LA JOLLA, CA 92093 USA
[5] TATA INST FUNDAMENTAL RES, BOMBAY 400005, INDIA
[6] MAX PLANCK INST PHYS & ASTROPHYS, D-8000 MUNICH 40, FED REP GER