COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     NUMBER 796

                                     Konkoly Observatory
                                     Budapest
                                     1973 May 21


      THE LARGE PERIOD VARIATION IN SS CAM EXPLAINED




    New UBV photoelectric photometry of the RS CVn-type eclipsing 
binary SS Cam, obtained during 1972 at Dyer Observatory and
Kitt Peak National Observatory, provides support for the theory
proposed by Hall (1972) to explain the large period variations
which are experienced by the RS CVn-type binaries. In this theory 
the continuous back and forth period variation is explained
as a result of continuous mass loss from an active (dark) region
on the equator of the cool star which differential rotation forces
to change its orientation in the binary system.
    Our 1972 UBV photometry, which will be published elsewhere
later, shows the minimum of the out-of-eclipse wave-like distortion 
to be located at orbital phase P = 0.56p. The 1913-1915 visual 
observations of McDiarmid (1917, 1924) show the minimum of the
wave at P = 0.27p. If these two phases are plotted versus time and
if the wave is assumed to have migrated towards decreasing orbital 
phase, as the theory requires, then we have a migration rate
diagram similar to that constructed by Hall (1972, Figure 2 ) for
RS CVn. From such a diagram we can deduce that the minimum of the
wave was located at P = 0.25p around 1917 and at P = 0.75p around
1956.
    The O-C diagram of SS Cam is plotted in this Bulletin, where
the residuals have been computed with the ephemeris

        JD (hel.) 2435223.580d + 4.82430d E

of Kreiner (1971). The last point is the one based on our UBV 
photometry and corresponds to a primary minimum at JD (hel.) 2441456.634.
The other points are based on times very kindly supplied to us by
Dr. J.M. Kreiner of the Krakow Observatory. The circled points are
normals representing several individual times.
    The arrow pointing down is the time when the minimum of the
wave was at P = 0.25p and the theory would predict that the mass-
losing active region was on the trailing hemisphere and should
causing the maximum rate of period decrease. The arrow pointing up

                           [FIGURE 1]

is the time when the minimum of the wave was at P = 0.75p and the theory 
would predict the maximum rate of period increase. Within the 
limitations imposed by this less than ideal O-C diagram, we would say
that the predictions of the theory are confirmed perfectly.




Dyer Observatory                      CLIFFORD N. ARNOLD
Vanderbilt University                 DOUGLAS S. HALL
Nashville, Tennessee 37235            ROBERT E. MONTLE
U.S.A.
May 10, 1973




References:

Hall, D.S. 1972, Pub.Astr.Soc.Pacific 84, 323. [BIBCODE 1972PASP...84..323H ]
Kreiner, J.M. 1971, Acta Astr. 21, 365. [BIBCODE 1971AcA....21..365K ]
McDiarmid, R.J. 1917, Astrophys. J.. 45, 50. [BIBCODE 1917ApJ....45...50M ]
McDiarmid, R.J. 1924, Princeton Univ. Contr. No. 7, 50. [BIBCODE 1924CoPri...7....1M ]