COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1382

                                   Konkoly Observatory
                                   Budapest
                                   1978 January 30




              SOME COMMENTS ON AB Cas



   AB Cas is one of the very interesting eclipsing systems known
to-day to have pulsating components. P. Tempesti (I.B.V.S. No.
596, 1971) announced for the first time the bright component of
AB Cas to be a Delta Scuti Star using many photoelectric 
observations.
   This star was observed photoelectrically at Gissar station
of the Astrophysical Institute in Dushanbe during two nights in
October, 1976 using the 70 cm reflector. The observations 
confirm the existence of a pulsating variable of Delta Scuti type
as the bright component, showing pulsations in the secondary
minimum (during the second night). The light amplitude in V
band of the UBV system and approximately the period of pulsations 
are the same as P. Tempesti has found.
   Carefully analysing the me an light curve of AB Cas in V
band shown in I.B.V.S., No. 596, 1971 we arrived at the following 
logical conclusion (unfortunately the present observations
are very few and are distributed during too short time interval).
   P. Tempesti has plotted all his observations during the
years 1967-1971 according to phases of the eclipsing binarity 
(orbital period is roughly equal to 1.3 days). If the pulsation
period (0.058d roughly) would not be strictly equal to P_orb/N
(N is a whole number), then we could not see any separated
pulsation cycles on the mean eclipsing curve of AB Cas. Indeed,
on the time basis of Tempesti's observations, containing roughly
25000 pulsation cycles, even such a small value of the deviation
delta=abs(P_puls-P_orb/N) as about 10^-6 days leads to the error in 
pulsation phases equal roughly to half of the value of the pulsation 
period. If so, we shall see on every orbital phase only
occasional (from minimum to maximum) values of magnitude of 
intrinsic light variability instead of more or less clearly separated 
pulsation cycles as P. Tempesti has shown in his paper.
    From this we can conclude that in the case of AB Cas the
strict synchronization of the pulsation period (and the beat one,
if it is present) with the orbital period must exist. If so, in
the case of beat phenomenon in this Delta Scuti star the mean
length of the main pulsation period and the value of beat period
must be extremely constant.
    Now one of us (M.S.F.) discovered the same effect in Y Cam.
Both pulsation periods (P_0 and P_beat) are indeed synchronized
(in the same sense) with the orbital one. The paper on Y Cam
will be published in complete form somewhat later.
    The synchronization (in both cases) of pulsation periods
with the orbital one means the synchronization with the rotation
period of the Delta Scuti component, if the rule of equality of
P_orb and P_rot is fulfilled in these binary systems. So, it 
probably indicates a non-radial pulsation of Delta Scuti stars.
    The authors are grateful to Dr. N.M. Shakhovskoj for fruitful 
discussion.







B.N. IRKAEV                    M.S. FROLOV
O.A. CHEKANIKHINA              Astronomical Council
Dushanbe Astrophysical         of the USSR Academy
Institute of Tadjik Academy    of Sciences
of Sciences, U.S.S.R.          U.S.S.R.