[1] ROYAL GREENWICH OBSERV,HAILSHAM BN27 1RP,E SUSSEX,ENGLAND
[2] STOCKHOLM OBSERV,S-13300 SALTSJOBADEN,SWEDEN
[3] RUTHERFORD & APPLETON LAB,ASTROPHYS GRP,DIDCOT OX11 0QX,OXON,ENGLAND
[4] OBSERV PARIS,F-92190 MEUDON,FRANCE
[5] UNIV ROME,IST ASTRON,I-00161 ROME,ITALY
[6] CNR,IST FIS COSMICA,I-20133 MILAN,ITALY