Cyclic period variability in SW Cyg

Analysis shows that, after decreasing until ~1919 and increasing since then, the period has now started decreasing again, with the inflection point of the O-C curve being ~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 effect, due to orbital motion around a third body, would require an unseen black hole companion of >=10M_{sun}_. A magnetic cycle operating in the K-type subgiant, an active dynamo star because of its convection and rapid rotation, is the best explanation. Addresses: Berrington R.C. & Hall D.S. hallxxds@crtvax.vanderbilt.edu Dyer Observatory, Vanderbilt University, Nashville, Tennesee 37235