COMMISSION 27 OF THE I. A. U.
                  INFORMATION BULLETIN ON VARIABLE STARS
                               Number 2475

                                                      Konkoly Observatory
                                                      Budapest
                                                      9 February 1984

                                                      HU ISSN 0374-0676



               PERIODIC LIGHT CURVE VARIATIONS OF RT LACERTAE



  The eclipsing binary RT Lac is one of the most interesting systems among
the RS CVn-type binaries. The photometric studies made in visual region so
far indicated the existence of distortions on the light curve. Milone (1976)
observed the system in the infrared region and suggested that it has an
infrared excess and UV deficiency. Besides, RT Lac is an active radio source
(cf. Gibson et al., 1978). RT Lac is known as an eclipsing binary which has a
sinusoidal distortion on its light curve. The distortion wave migrates towards
decreasing orbital phase as in other RS CVn-type binaries. Hall and Haslag
(1976) estimated a period of about 9.5 years for the migration wave. They
also pointed out that the amplitude of the wave was variable ranging from
0.17 to 0.01 magnitude. The star spot cycle of about 30 years has been
estimated from the amplitude variations.

                             [FIGURE 1]

 Figure 1   The variation of the brightness of the system at second
 quarter in yellow and blue light


  The system has been observed photoelectrically in successive five years
   1978 to 1983 at the Ege University Observatory. The observations and the
almost complete light curves obtained between 1978 and 1981 were published
by Ibanoglu et al. (1980) and by Tunca et al. (1983). The orbital period of
the system was found to be decreasing with an amount of 34 s/century and
given in the last paper.
  The brightness of the system at second quarter and in primary eclipse were
plotted against the years in which the observations were carried out and are
shown in Figure 1, and 2. The light variations at second quarter show a smooth

                            [FIGURE 2]

 Figure 2   The variations of the brightness of the system at mid-primary 
 in yellow and blue light


curve and produce a period of about five years. Similar variations are also
seen in the brightness of the system at primary eclipse, but the period is


about four years. Both of the variations at second quarter and at primary
eclipse are similar in the colours V and B. The observations of the system
RT Lac obtained in successive five years suggest that the light curve variations
are periodic with a period of about 4.5 years. This value is nearly
the half of that proposed by Hall and Haslag (1976).





         S. EVREN, C. IBANOGLU, Z. TUNCA and O. TUMER

              Ege University Observatory
              Bornova-IZMIR/TURKEY





References:

Gibson, D.M., Owen, F.N. and Hjelming, R.M.: 1978, Publ.Astron.Soc. Pacific
  90, 751 [BIBCODE 1978PASP...90..751G ]
Hall, D.S. and Haslag, K.P.: 1976, in I.A.U. Coll. No. 29, Multiple Periodic
  Variable Stars, Part II, P. 331 [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 ]
Tunca, Z., Ibanoglu, C., Tumer, O., Ertan, A.Y. and Evren, S.: 1983,
  Astrophys. Space Sci. 93, 431 [BIBCODE 1983Ap&SS..93..431T ]