COMMISSION 27 OF THE I.A.U.
                  INFORMATION BULLETIN ON VARIABLE STARS
                              Number 2918

                                                         Konkoly Observatory
                                                         Budapest
                                                         23 July 1986

                                                         HU ISSN 0374 - 0676


             BEHAVIOUR OF AT CANCRI IN THE SEASON 1985/86

The cataclysmic star AT Cancri was measured on 46 blue-sensitive
plates (ORWO-ZU21-GG13+BG12) from 12 nights obtained with the 50/70/172 cm
Schmidt camera of Sonneberg Observatory covering the time interval between
17 November 1985 and 12 April 1986, using the sequence of comparison stars
given in the IBVS No. 2363. The observations are listed in Table I.


                             Table I

J.D.hel      m_B       J.D.hel      m_B        J.D.hel      m_B
244..                  244....                 244..

6387.596    12.37      6469.480    15.52       6476.449    12.35
6387.616    12.32      6469.499    15.98       6476.472    13.22
6387.635    12.34      6469.518    15.34       6476.495    12.81
6387.654    12.42      6469.537    15.84:      6476.523    12.92
6387.673    12.35      6469.556    15.49       6489.361    15.58
6440.488    15.79      6473.542    16.36:      6491.368    12.49
6440.507    15.67      6473.563    16.38:      6491.388    12.44
6440.526    14.95      6474.358    15.98       6491.407    12.57
6463.381    12.29      6474.378    15.87       6491.426    12.46
6466.401    12.78      6474.399    15.69       6491.445    12.54
6469.356    15.67      6474.419    15.10       6506.518    13.02
6469.377    15.57      6474.438    15.83       6533.364    14.74
6469.397    15.13      6474.463    16.38       6533.384    14.61
6469.419    15.03      6474.483    16.25       6533.404    14.73
6469.440    15.51      6476.429    12.89       6533.423    15.02
6469.460    15.48


   The long-term light curve of AT Cnc, which is given in Figure 1,
shows variations between m_B=16.38 and m_B=12.29. Some remarkable changes
in brightness were observed with Deltam_B=+2.89 within 2.955 between 4
February and 7 February and Deltam_B=-3.09 within 2.071 and 2.007
respectively between 12 February and 14 February and between 27 February and 1
March 1986.

                        [FIGURE 1]
                        [FIGURE 2]
                        [FIGURE 3]

   The brightness distribution of the star from all series of observation
given in the IBVS No. 2363, No. 2526, No. 2734 and in Table I is shown in 
Figure 2. It can be seen there that AT Cnc prefers two states of brightness,
the high (m_B=~12.75) and the low (m_B=~15.75) one.
   Small short time-scale variations of the star can be observed in all
series. They are regular and refer to orbital light changes. The observations
from the season 1985/86 confirm this statement which was first announced in
the IBVS No. 2734. Reducing all observations from all series to one common
epoch the preliminary orbital elements given there could be improved to

              Min.hel.= 244 6110.504 + 0.2386913d * E.

                          [FIGURE 4]

   The results are given in Figures 3 and 4 where the magnitudes m_B from
observations of the low (14.5 < m_B <16.4) and the high (12.3 < m_B < 14.5)
state of brightness obtained between the years 1982 and 1986 are plotted
against the phases. The mean values are marked by crosses there. Comparing
Figures 3 and 4 it can be seen that in the high state the minimum phase is
displaced to phase ~0.5 rather than phase 0.75 as was provisionally stated in
the IBVS No. 2734.


                                W. GOTZ
                       Akademie der Wissenschaften
                       der DDR, Zentralinstitut für
                     Astrophysik, Sternwarte Sonneberg