COMMISSION 27 OF THE I. A. U.
	 INFORMATION BULLETIN ON VARIABLE STARS
		     Number 2655


			          Konkoly Observatory
			          Budapest
			          21 January 1985

			          HU ISSN 0374 - 0676



	    THE LIGHT CHANGES OF RT LACERTAE



 The eclipsing system RT Lac is known as an active binary among the RS CVn
type stars. The behaviour of the system is very interesting both in optical and
radio regions. The existence of sinusoidal distortion on its light curves has
been subject to various investigations (cf. Ibanoglu et al. 1980, Milone 1976,
1977, Tunca et al. 1983). Examining all available light curves obtained between
1890 and 1970, Hall and Haslag (1976) estimated a period of about 9.5 years for
the migration of the wave. They have also derived a star spot cycle of about 30
years from the variations of the amplitude of the wave.

                          [FIGURE 1]

 Figure 1: The primary eclipses obtained in July and September 1984.

                          [FIGURE 2]

 Figure 2: The variations of the mean brightness of the system at second 
quarter in yellow and blue light.

                          [FIGURE 3]

 Figure 3: The variations of the mean brightness at mid-primary in blue and
yellow light.

                         
Recently, Evren et al. (1984) published their results obtained from the
photometry during last six years. The light variations during primary eclipse
and also at second quarter show a similarity according to their investigation.
But the period found from light variations at primary eclipse is shorter then
that of second quarter. The mean period of about 4.5 years for the light 
variation has been suggested.
The system has been observed again in 1984 observing season. The observations
during primary eclipse obtained on July 21, 26 and on September 25 have been
plotted versus orbital phase and are shown in Figure 1. It is clearly seen that
the brightness of the system has been changed within each orbital revolution.
The observations obtained at the end of September show that the depth of the
primary eclipse has been started to increase again. The mean brightnesses at
second quarter and at primary eclipse have been also plotted in Figure 2 and 3.
Figure 3 shows that the brightness of the star seen at the primary eclipse has
been starting to decrease. We estimate that it will reach its minimum brightness
about 1986 observing season.
Results of an international observing program at different wavelengths, which
will be carried out in the next two years, of this peculiar system will of course
yield us valuable information about RT Lac.





			       C. IBANOGLU, S. EVREN,
			       Z. TUNCA, and O. TUMER
			       Ege University Observatory
			       Bornova - Izmir/Turkey


References:
Evren, S., Ibanoglu, C., Tunca, Z., and Tumer, O.; 1984, IAU, Inf.Bull.Var.Stars. No. 2475.
Hall, D.S., and Haslag, K.P.: 1976, 'Multiple Periodic Variable Stars', IAU
          Colloq.  29. [BIBCODE 1976mpvs.conf..331H ]
Ibanoglu, C., Kurutac, M.,  Tumer, O., Evren, S., Tunca, Z., and 
          Ertan, A.Y.; 1980, Astrophys.Space  Sci. 72, 61. [BIBCODE 1980Ap&SS..72...61I ]
Milone, E.F.: 1976, Astrophys.J.Suppl. 31, 93. [BIBCODE 1976ApJS...31...93M ]
Milone, E.F.: 1977, Astron.J. 82, 998. [BIBCODE 1977AJ.....82..998M ]
Tunca, Z., Ibanoglu, C., Tumer, O., Ertan, A.Y., and Evren, S.: 1983, 
          Astrophys. Space Sci. 93, 431. [BIBCODE 1983Ap&SS..93..431T ]