COMMISSION 27 OF THE I. A. U.
      INFORMATION BULLETIN ON VARIABLE STARS
                  NUMBER 200


                                  Konkoly Observatory
                                  Budapest
                                  17 May 1967




        THE BEAT-PERIOD IN THETA OPHIUCHI


     The first systematic photoelectric observations of Theta Oph
were made in 1958 on A. VAN HOOF's request, with the 1,50 m -
telescope of the Boyden Observatory at Bloemfontein, by Dr. H.
HAFFNER, G. LYNGA and L. O. LODEN. All their observations of the
variable Theta Oph and of the comparison star 44 Oph were made in
ultraviolet light. Prof. A. VAN HOOF published the first photoelectric
light curves of this Beta CMa star. He mentioned a short period P0=
0.140531day and a beat period of about 6 days.

     In 1966, we intended to start with a new search about these
periods. The variable star Theta Oph was observed very intensively
in 1966, during the months June, July and August, with the same 
instruments in 1958. Our photoelectric observations were carried
out in three colours: U, B and V.  The light curves derived, by means
of the comparison star HD 158643, show very clearly the variability
of the amplitude from night to night.
 
     The short period cycle of the light variations has the value
P0 = 0.140531day as VAN HOOF (1) found in 1958. This value fits our
observations very well, but the amplitude seems to change with a
period of about 4 days.

     The light curves presented in this paper are a selection out
of 23 ones obtained in 1966.

                            [FIGURE 1]

     The differences between maximum and minimum light 
obtained from the ultraviolet light curves are presented in Table 1. We
are only giving the values out of two intensive observation periods.


Table 1.


                         Min.                 Max.


JD 2439309 Delta m_u(O-C) 0.030m
       310                       .032
       311                                   .036


       313               .032


       315                                   .046
       316                       .043
       317               .030


       319                                   .036


       336               .026
       337                       .037
       338                                   .042
       339                       .035
       340               .015
       341                       .027
       342                                   .035
       343                       .034


      It is possible to indicate the beat period of 4 days also in
Fig. 1 of VAN HOOF' s paper (1).

      In Table 2, we are giving our estimates of the amplitudes
from his light curves.



Table 2


JD 2436380 Delta m_u= 0.04m    Max.
       382            .02      Min.


       413            .02      Min.
       414
       415            .04      Max.


      The best value of the beat period that fits all these 
observations is: 
                      Pb = 3.9d..;

      From the results in Table 1 we see that the values of the
amplitudes in the maxima and in the minima are not constant. They
seem to change with another long period of about 16 days (maybe
4 x 3.9d = 15.6d).

      Acknowledgments are due to the Belgian Ministry of National
Education and Culture, "Nationaal Fonds voor Wetenschappelijk
Onderzoek", for the grant that made possible our stay at the Boyden
Observatory, Bloemfontein, South-Africa.


      May 10, 1967.                         Remy BRIERS
                                       Astronomisch Instituut
                                               Leuven
                                              Belgium


(1) VAN HOOF,  A. Zs. f. Astroph. 54, 255 (1962).