COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1862

                                   Konkoly Observatory
                                   Budapest
                                   1980 November 12

                                   HU ISSN 0374-0676





      A SUDDEN ACCELERATION IN THE MIGRATION RATE OF RS CVn

  The purpose of this note is to show that our recent photometry of the wave
in RS CVn, along with previously published values of the phase of wave minimum,
seems to indicate a recent sharp acceleration in the migration rate and a
simultaneous decrease in the amplitude of the wave.
  Eaton obtained observations in V only with the 24-inch reflector at the
Pennsylvania State University Observatory between JD 2443945.74 and 2444001.66.
Fourier analysis of his light curve outside eclipse yielded Delta V = 0.189m+-0.020m
for the wave amplitude and Phi(min.) = 0.871p +- 0.017p for the phase of its mini-
mum. Henry obtained observations in V and B with the No. 4 16-inch at Kitt
Peak National Observatory and the 24-inch at Dyer Observatory between JD 2444316.86
and 2444425.64. Fourier analysis of his light curves outside eclipse yielded
Delta V = 0.116m +- 0.009m and Delta B = 0.097m +- 0.007m for the wave amplitude and 0.510p +- 0.009p
and 0.511p +- 0.008p for the phase of the wave minimum in V and B, respectively. In
this Fourier analysis, phases were computed with the ephemeris of Evren et al.
(1980) The equipment and observing technique used by Eaton and Henry have been
described already by Burke et al. (1980).

   In the Figure below, the top part is the migration curve between 1956 and
1980, with the ordinate Phi(min.) being the phase of wave minimum. The three
squares are from Popper (Hall 1972); the eleven points are from Catania
(Catalano et al. 1980); the two crosses are from Oliver (1975); the two plusses
are from Ludington (1978); the one open circle is from Eaton (this paper); the

                       [FIGURE 1]



one circled point is from Evren et al. (1980); and the one filled circle is
from Henry (this paper). In the bottom part, each symbol is the corresponding
residual


             Delta Phi(min.) = Phi(min.) - Phi'(min.),

where Phi'(min.) is the phase of wave minimum computed with the 9.48-year migration 
period determined by Catalano et al. from the Catanian observations
between 1963 and 1973. Specifically

            Phi'(min.) = 0.45p - (T - 1963.32)/9.48,

where T is the mean epoch of each light curve.
   In the Delta Phi(min.) plot we see that a migration period of 9.48 years does
indeed fit the observations very well between 1963 and 1973, the horizontal
straight line segment. On the other hand, the migration rate was much slower
in the late 1950's and now, as of 1980, is apparently much faster. A fit
to the earliest three symbols would indicate P(migr.) ~20 years whereas a
fit to the latest three symbols would indicate P(migr.) ~ 3 years.
   Because another point or two in the figure is needed to define the
recent trend better, it would be premature to speculate now on physical interpretations. 
We should, however, issue the following warning immediately.
If the migration rate really has accelerated recently, then we are in danger
of losing continuity in the migration curve, which has been maintained now
for the 24 years since 1956. Anytime the wave migrates an appreciable
fraction of one cycle between one light curve and the next, >~ 0.4p for example,
we cannot know with confidence whether it moved towards increasing or towards
decreasing phase. Therefore we urge observers of RS CVn binaries (and
observers of RS CVn itself in particular) to begin observing RS CVn as soon
as it becomes available in the eastern sky and continue photometry throughout
the observing season, thereby obtaining not just one but rather two or perhaps
three light curves for the year 1981.



   It is of additional interest to note that, whereas the wave amplitude
(max. to min.) has remained around 0.2m in V throughout the 1960's and
1970's, the most recent three light curves show it decreasing to about half
that during the course of only one year. Eaton found Delta V = 0.19m+-0.02m in
1979.27, Evren et al. found Delta V = 0.14m in 1979.41, and Henry found Delta V = 0.12m+-0.01m
in 1980.36. It would be very interesting if we are seeing the beginning of a
decline from a spot cycle maximum to spot cycle minimum.
   D.S.H. is happy to acknowledge that part of this work was supported
by N.A.S.A. research grant NSG-7543.


                             JOEL A. EATON
                             DOUGLAS S. HALL
                             GREGORY W. HENRY*
                             Dyer Observatory
                             Vanderbilt University
                             Nashville, Tennessee 37235








References:


Burke et al. 1980, A. J. 85, 744. [BIBCODE 1980AJ.....85..744B ]
Catalano, S., Frisina, A., Rodono, M. 1980, I.A.U.
    Symposium No. 88, 406. [BIBCODE 1980IAUS...88..405C ]
Evren, S. et al. 1980, I.B.V.S. No. 1732.
Hall, D. S. 1972, P.A.S.P. 84, 323. [BIBCODE 1972PASP...84..323H ]
Ludington, E. W. 1978, Ph.D. Thesis, University of
    Florida, Gainesville, Florida. [BIBCODE 1978PhDT.......103L ]
Oliver, J. P. 1975, P.A.S.P. 87, 695. [BIBCODE 1975PASP...87..695O ]


* Guest Investigator, Kitt Peak National Observatory, which is operated by the
Association of Universities for Research in Astronomy, under contract with
the National Science Foundation.