Spin-up in SMC X-1.

Results are reported for 44 separate measurements of the pulsation period and the average light curve of SMC X-1 made over a complete binary cycle with instruments aboard the Ariel V satellite. The eclipse semiduration is estimated to be about 0.310 day, a spectral slope of approximately -1.3 is determined, and a best-fit pulsation period of about 0.1747337 sec is obtained. A comparison with a previous measurement by the SAS 3 satellite implies a period change of 3.7 microsec per binary period, corresponding to a spin-up rate of approximately -1.55 by 10 to the -11th power per sec. The phase zeros given by the Ariel V and SAS 3 measurements are used to derive a binary period of about 3.89229 days, which is free of any possible systematic error due to uncertainty in measuring an eclipse center from the geometrical eclipse.