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 0)./sub /787313 period derived by Gottlieb, Wright, and Liller. The double period light curve, folded at 1)./sub /574626, 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. These are described, and we conclude by making recommendations for further observations