COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2033    

                                 Konkoly Observatory
                                 Budapest
                                 1981 October 30

                                 HU ISSN 0374-0676





  THE PRINCIPAL FREQUENCY OF THE RAPIDLY OSCILLATING
                 Ap STAR alpha Cir



  We have obtained 47 hours of high speed photometric observations 
of the rapidly oscillating Ap star alpha Cir (Kurtz and Cropper
1981) with the 0.5-m, 0.75-m and 1.0-m telescopes of the South
African Astronomical Observatory. A frequency analysis of these
observations indicates that most of the light variability in
alpha Cir can be described by a single frequency
f = 8.79130 + 0.00012 hour^-1 (P = 6.82493 + 0.00009 minutes) with
a semi-amplitude of 1.9 m mag. Figure 1 shows the fit of that
frequency to the light curve obtained on the night of 1981 June
08/09.
  It is also clear that the amplitude of the principal frequency
in alpha Cir is variable, but we have insufficient data to derive
the secondary frequencies. In terms of the oblique pulsator
model (Kurtz 1981) it should be possible to derive the rotation
period of alpha Cir and place constraints on iota, the inclination of
the rotation axis to the line of sight, and beta the inclination
of the magnetic axis to the rotation axis, once the secondary
frequencies and their amplitudes are known.   


                 D.W. KURTZ
                 S. ALLEN
                 M.S. CROPPER
                 Department of Astronomy
                 University of Cape Town
                 South Africa

References:
Kurtz, D.W., 1981 Mon. Not. R. astr. Soc., in press. [BIBCODE 1982MNRAS.200..807K ]
Kurtz, D.W., and Cropper, M.S., 1981, I.B.V.S., No. 1987.


                          [FIGURE 1]

 Figure 1: The fit of the principal frequency to the light curve for the
night of 1981 June 08/09. The light curve should be read like lines of print
with the right edge of each panel connecting directly to the left edge of
the panel beneath. Each panel is 1 hour long and 0.04 mag high. The data
points represent 20-s integrations. In comparing the solid line fit to the
data points, one should compare only the amplitude and phase. Small vertical
shifts of sections of the light curve (as in the bottom panel) are due to sky
transparency variations and should be ignored.