COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                       Number 3453

                                              Konkoly Observatory
                                              Budapest
                                              26 April 1990

                                              HU ISSN 0374 - 0676


             NO MAJOR PERIOD CHANGE FOR CT TAURI

  Recently, Wlodarczyk and Drozdz (1990) reported photoelectric observations
of the B2 + B2 contact system CT Tau, P = 0.6668d. From their times of primary
and secondary minima, spanning 1.5 orbital cycles, they found an extraordinary
period increase of 1.47 minutes. This result appeared to be confirmed by a
photoelectric time of minimum earlier in the same season by J. Kaluzny, reported
to them in a private communication.
  However, numerous recent visual timings of minima do not support this period
change. Figure 1 is an O-C plot of minima timings published by the Eclipsing
Binary Observers of the Swiss Astronomical Society (BBSAG 1972 et seq.) and
unpublished timings in the files of the AAVSO Eclipsing Binary Committee. The
O-C residuals were calculated using the light elements in the General Catalogue
of Variable Stars (Kholopov et al. 1985).
  The diagram shows that the photoelectric timings by Wlodarczyk and Drozdz
and the visual timings are consistent, while Kaluzny's time of minimum is
discordant. Perhaps a typographical error occurred in the communication of this
timing, or a calculating error in the decimals of a Julian Day.
  Least-squares analysis of the 39 visual timings after JD 2443000 results in
a period identical with the GCVS value, 0.6668303d, but indicates that the
initial epoch of the GCVS elements requires a small correction. The
photoelectric time of primary minimum by Wlodarczyk and Drozdz is within 0.001d
of the time predicted by this analysis, so revised light elements can be based
on this timing and the period confirmed by the visual data:.

           Min I = HJD 2447553.5464 + 0.6668303d E                 (1)
                               +/-4        +/-8



                             [FIGURE 1]

        Figure 1. CT Tau, O-C diagram calculated from the GCVS
        light elements Min. I = HJD 2445404.359 + 0.6668303d E.




   The remarks section of the GCVS notes that the period of CT Tau is variable.
Chugainov (1965) found P = 0.6668276d beginning in 1952, and Ahnert (1965) found
P = 0.6668274d, a virtually identical value for 1948-1965. Figure 1 also
suggests that the period was shorter than the current value before JD 2443500.


       DAVID B. WILLIAMS                 MARVIN E. BALDWIN
       9270-A Racquetball Way            Route 1
       Indianapolis, IN 46260 USA        Butlerville, IN 47223 USA


REFERENCES

Ahnert, P., 1965, MVS 2, H. 7, 181.
BBSAG, 1972 et seq., BBSAG Bulletin Nos. 2, 6, 8, 11, 17, 18, 19, 20, 21, 25,
   26, 27, 31, 32, 39, 42, 47, 58, 64, 65, 66, 87.
Chugainov, P. F., 1965, Inform. Bull. Var. Stars No. 95.
Kholopov, P. N., editor, et al., 1985, General Catalogue of Variable Stars,
   Nauka, Moscow.
Wlodarczyk, K., and Drozdz, M., 1990, Inform. Bull. Var. Stars No. 3421.