COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 2734

                                           Konkoly Observatory
                                           Budapest
                                           27 May 1985

                                           HU ISSN 0374 - 0676



         OBSERVATIONS OF AT CANCRI IN THE SEASON 1984/85



  The star AT Cnc, which is probably of cataclysmic type, was measured on
66 blue-sensitive plates (ORWO-ZU21+GG13+BG12) from 14 nights obtained with
the 50/70/172 cm Schmidt camera of Sonneberg Observatory covering the time
interval between 29 November 1984 and 22 March 1985, using the sequence of

                    Table I

J.D.hel        mB      J.D.hel       mB       J.D.hel       mB
244....                244....                244....
6034.595      15.80mag 6108.411      15.45mag 6113.390       16.20mag
6034.616       16.00   6108.431       15.40   6113.411       15.60
6034.635       15.65   6108.454       15.40   6113.431       15.80
6034.656       15.95   6108.474       15.40   6113.474       15.50
6034.677       15.55   6109.372       15.40   6113.495       15.55
6036.693       14.75   6109.390       15.45   6116.333       13.70
6036.711       14.85   6109.408       15.40   6116.354       13.70
6083.573       15.05   6109.429       15.20   6116.372       13.45
6083.639       15.40   6109.449       15.30   6116.392       13.80
6083.662       15.40   6109.469       15.65   6116.411       13.85
6083.683       15.35   6110.351       15.20   6116.430       14.00
6084.419       15.85   6110.373       15.10   6116.451       13.75
6084.436       15.90   6110.393       15.20   6121.397       13.30
6093.507       13.15   6110.431       15.60   6121.417       13.35
6093.527       13.10   6110.449       15.75   6121.437       13.25
6093.547       12,95   6110.471       16.00   6121.457       13.45
6093.567       12.80   6110.493       16.05   6121.479       12,90
6093.586       12.90   6110.603       15.70   6137.347       13.10
6093.630       13.10   6111.449       16.00   6137.368       13.20
6108.347       15.80   6111.470       15.50   6147.322       14.45
6108.366       15.95   6113.351       16.15   6147.342       14.25
6108.386       15.75   6113.370       16.05   6147.362       14.10


                            [FIGURE 1]
                            [FIGURE 2]


comparison stars given in the IBVS No. 2363. The observations are listed in
Table I. In order to study the short-term behaviour of the star, more than
one plate were obtained in all nights. The exposure time of the plates
amounts to 20 minutes.


                            [FIGURE 3]


The long-term light curve of AT Cnc which results from the mean magnitudes
of the nights listed in Table I, given in Figure 1, shows variations between
mB=15.90mag and mB=13.00mag. A remarkable increase in brightness with DeltamB=-2.09mag
within 2.97d was observed between 16 February and 19 February 1985. Small
short time-scale variations of the star can be observed in all series. They
seem to be regular and refer to occultation light changes. In order to study
the influence of these variations all observations were reduced to one common
epoch by means of the preliminary orbital elements:

       Min. Hel. = 244 6110.504 + 0.238 6556d E
  The results are given in Figures 2 and 3 where the magnitudes mB from
observations of the low (15.0mag<mB<16.2mag) and the high (12.6mag<mB<13.5mag)
state of brightness are plotted against the phases. In Figure 3 the series
of observations from the night J.D. 244 5672 given in the IBVS No. 2526 (1984)
and marked by circles are also drawn. Comparing Figures 2 and 3 it can be
seen that in the high state the minimum phase is displaced to phase ~0.75.
More observations are still needed to make further and detailed statements
about the behaviour of AT Cnc.




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