THE ASTRONOMICAL JOURNAL    VOLUME 107, NUMBER 5, PAGE 1868            MAY 1994

                   THE CYCLIC PERIOD VARIABILITY IN SW CYGNI

                   ROBERT C. BERRINGTON AND DOUGLAS S. HALL

      Dyer Observatory, Vanderbilt University, Nashville, Tennesee 37235
                Electronic mail: hallxxds@crtvax.vanderbilt.edu

                                   ABSTRACT
    
    Timings of primary eclipse up through 1992 are collected, plus a few
    overlooked in earlier studies.  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 >= 10 Msun.  A magnetic cycle operating
    in the K-type subgiant, an active dynamo star because of its convection
    and rapid rotation, is the best explanation.