COMMISSION 27 OF THE I. A. U.
             INFORMATION BULLETIN ON VARIABLE STARS
                          Number 3421

                                                 Konkoly Observatory
                                                 Budapest
                                                 14 January 1990

                                                 HU ISSN 0374 - 0676

                     NEW PERIOD OF CT TAURI

  The eclipsing system CT Tau containing stars of B2 spectral type 
belongs to the small group of contact binaries with hot components. This 
unusual system has not been analysed in detail yet, so we decided to observe
it.
  The observations in B and V Johnson passbands were made on the 0.6 m
telescope at Mt. Suhora Astronomical Observatory. We used the double beam
photometer donated by ESO thanks to initiative of Prof. E.H. Geyer. This
photometer was described by Szymanski and Udalski (1989).
  We obtained BV light curves during two consecutive nights 25/26 and
26/27 January 1989. In observing runs BD+26 917 was used as the comparison
star. The integration times in both filters were fixed at 20 seconds.
  During the final reduction we found that the present observations
could not be phased with the period P = 0.6668303 determined in 1983 and
given in GCVS. So, we have examined the data with the Kwee method to determine 
the times of minima and the new orbital period. The times of minima
calculated from B and V light curves are slightly different.
For B we obtained:
  minI  = HJD 2447553.5486 +/- 0.0004
  minII = HJD 2447552.5462 +/- 0.0003

and for V:
  minI  = HJD 2447553.5443 +/- 0.0004
  minII = HJD 2447552.5432 +/- 0.0008
  These differences are caused by small asymmetry of eclipse profiles.
To calculate the new orbital period the mean times of minima were used. We
obtained the following linear ephemeris:
  minI = HJD 2447553.5464 + 0.6678537d * E
The observations phased with the new ephemeris are presented in Figure 1.
Our result was strongly confirmed by Kaluzny (1989) who found minI =


                             [FIGURE 1]

Figure 1. The observations of CT Tau in B filter phased with the new
      ephemeris


HJD 2447507.4622 in December 1988. So, we can conclude that the period of
CT Tau has increased by 1.47 minutes during the last six years. This 
indicates that the rate of period change P = 4.76 * 10E-7.
         
  This work was carried out under the Polish Research Program RPBR I-II.



                           K. WLODARCZYK
                           M. DROZDZ

                    Mt. Suhora Astronomical Observatory
                    Pedagogical University, Podchorazych 2
                    30-084 Cracow, Poland




References:
Kaluzny, J.: 1989, private communication.
Szymanski, M., Udalski, A.: 1989, Acta Astr., 39, 1. [BIBCODE 1989AcA....39....1S ]