[1] MIT,DEPT SPACE RES,CAMBRIDGE,MA 02139
[2] HARVARD SMITHSONIAN CTR ASTROPHYS,CAMBRIDGE,MA 02138
[3] EUROPEAN SPACE AGCY,EUROPEAN SPACE RES & TECHNOL CTR,EXOSAT OBSERV,DEPT SPACE SCI,2200 AG NOORDWIJK,NETHERLANDS
[4] MAX PLANCK INST EXTRATERR PHYS,D-8046 GARCHING,FED REP GER