Optical studies of Uhuru sources. XII. The light curve of Scorpius X-1 = V818 Scorpii, 1889 - 1974.

An analysis of 1068 photographic observations of V818 Sco = Sco X-1 from the Harvard plate collection, as well as 698 post discovery magnitudes, shows that its B magnitude ranges between 11.8 and 13.7. However, there is strong evidence that Sco X-1 has been systematically fainter by 0.3-0.4 mag since the mid-1960s. All the available magnitudes have been combined to give an improved mean light curve at the .787313-day period derived by Gottlieb, Wright, and Liller. The double period light curve, folded at 1.574626 days, is also given and discussed. The radial velocity curve, based on published data, and the variations in color make it possible to draw new conclusions on plausible models for Sco X-1.