COMMISSION 27 OF THE I. A. U.
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3087


                                           Konkoly Observatory
                                           Budapest
                                           17 September 1987

                                           HU ISSN 0374-0676


          LIGHT CHANGES IN THE PRIMARY ECLIPSE OF RT Lac


   Photoelectric observations of the active RS CVn type binary RT Lac have
been continued at the Ege University Observatory. In this note we report about
our observations and the light changes within the primary eclipse. The 
observations were carried out with the 48 cm Cassegrain reflector and one channel
B, V photoelectric photometer. A photon counting system, Starlight 1, with an
integration time of 10 sec was used. A microcomputer, Amstrad CPC6128, controls
the photometer. An uncooled, standard EMI 9924A photomultiplier tube
was used at the observations.

   A primary eclipse was observed on 27/28 July 1987 in two different 
wavelengths, in blue and yellow. These are shown in Figure 1. The light 
variations have been discussed by Ibanoglu et al, 1980) and the periodic light
changes within primary eclipse have been reported by Evren et al. (1984). A
period for the light variation at mid-primary of about 4.5 years has been 
suggested. Later on, Ibanoglu et al. (1985) announced that the brightness of the
star seen at the primary eclipse had been started to decrease and it would
reach its minimum brightness about 1986 observing season. The mean values of
brightness at mid-primary have been plotted against the years and are shown
in Figure 2. It is clearly seen that the brightness at mid-primary reached
its minimum value, which is the same as that in 1981 in blue, but it is a
little brighter in yellow light. The last observations of the primary eclipse
clearly confirmed that the light variation at mid-primary eclipse is periodic
and is repeated with a period of about 5 years.
   The light variations outside minima are nearly half of that seen at the
primary. Therefore, the events observed in RT Lac cannot be explained with
the spot hypothesis alone. These existing light variations in mid-primary
eclipse of RT Lac clearly reveal that the component seen during the primary
eclipse could be an intrinsic variable.


                               [FIGURE 1]


  Results of an international observing program at different wavelengths,
which will be carried out in this and next observing seasons, of this peculiar
system will of course yield valuable information about RT Lac.


                               [FIGURE 2]




                     V. KESKIN, S. EVREN, C. IBANOGLU

                        Z. TUNCA and M.C. AKAN
                     Ege University Observatory
                        Bornova-Izmir/TURKEY


References:

Evren, S., Ibanoglu, C., Tunca, Z., and Tumer, O.: 1984, IAU 
      Inf.Bull. Var.Stars No. 2475.
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 ]
Ibanoglu, C., Evren, S., Tunca, Z., and Tumer, O.,: 1985, IAU 
      Inf.Bull. Var.Stars No. 2655.