SPECTROSCOPIC AND PHOTOMETRIC-OBSERVATIONS OF SUPERNOVA 1987A .7. DAYS 793 TO 1770

We present spectroscopic and UBV(RI)(C)JHKLM photometric observations of SN 1987A in the Large Magellanic Cloud made between days 793 and 17 70 after the Kamiokande-II neutrino event, as well as some CCD UBV(RI) C photometry from day 640 to day 792, which overlaps the interval cove red in the preceding paper of this series. During the period from abou t day 670 to day 1260, the U- to M- bolometric magnitude (hereafter M( UM)) of the supernova and its adjoining circumstellar ring (hereafter 'the enclosed supernova') dimmed at an increasingly slower rate than d uring the preceding interval from day 500 to day 670. The rate of dimm ing steadily increased again between days 1270 and 1770. From day 1000 onwards, the light of the circumstellar ring made an increasingly sig nificant contribution to the light of the enclosed supernova, and by d ay 1250 had become so dominant that it is probable that useful Earth-b ased photometry of the bare supernova itself was not feasible thereaft er.