The Cyclic Period Variability in SW Cygni
Timings of primary eclipse up through 1992 are collected, plus a few overlooked in earlier studies. Analysis show that, after decreasing until approximately equals 1919 and increasing since then, the period has now started decreasing again, with the inflection point of the O-C curve being approximately 1972. The shortest period was 4.5727 days, the longest 4.5732 days, with a full cycle length of 96 yr. Explanation by the time-delay efffect, due to orbital motion around a third body, would require an unseen black hole companion of greater than or equal to 10 solar mass. A magnetic cycle operating in the K-type subgiant, an active dynamo star because of its convection and rapid rotation, is the best explanation.