COMMISSION 27 OF THE I.A.U.
                 INFORMATION BULLETIN ON VARIABLE STARS
                             Number 3252

                                                         Konkoly Observatory
                                                         Budapest
                                                         20 October 1988

                                                         HU ISSN 0374 - 0676


                      SLOW VARIABILITY OF V1500 Cyg

Regular photometric observations of Nova Cygni 1975 (V1500 Cyg) have been
continued in the Crimean Observatory from 1975 up to now. The 0.5 meter 
meniscus telescope equipped with a TV system is used to get estimates of the
brightness close to the V photometric band. The large amount of data
obtained so far (2150 points) makes it possible to use statistical methods
for searching for possible regularities in the light curve of V1500 Cyg
in addition to the well-known main period P =0.13961325d (the corresponding
phase dispersion spectrum is displayed in Figure 1).
   We used the classical method of least squares approximation to obtain
parameter estimates for polynomial trend (taking into account the fading of
Nova) and harmonic components of the main period. After that we computed
O-C residuals from the estimated model. In Figure 2 low resolution phase
dispersion spectrum of residuals is plotted against frequency. The strongest
minimum in the region under study (P=5-20 days) corresponds to P_1=7.70d.
However, the refined spectrum for the region around P_1 (Figure 3) shows 
several peaks (P1=7.7019d, P2=7.7352d, P3=7.7731d etc.) and consequently we 
cannot speak about the feature around P=7.70 as the strict period.
   G. Schmidt and H.S. Stockman (IAU Circ. No. 4458) studied the polarization 
of V1500 Cyg and found that polarization is variable with the period
of P_pol=0.1376d. Within the range of permissible errors the following near
equality holds:

                P^-1_pol=P^-1_0+P^-1_1

and it cannot be ruled out that P_1 is the "beat" period between P_pol and P_0.
   If P_pol is the period of rotation for the white dwarf (as was conjectured
by G. Schmidt and H.S. Stockman) and P_0 is the period of its revolution
in the system, then P_pol must synchronize with P_0 in the future and the
period P_1 must change as well. Then the appearance of fine phase dispersion 
spectra can be attributed to the interplay between the changing nature
of P_1 and random distribution of observations in time.


                          [FIGURE 1]

      Figure 1: Crude PDM spectrum for detrended data.

                          [FIGURE 2]

        Figure 2: Crude PDM spectrum for O-C residuals.

                          [FIGURE 3]

    Figure 3: Refined PDM spectra for O-C residuals, sequential analysis


   Evaluation of statistical significance for the obtained results is rather
cumbersome because of the complex nature of V1500 Cyg variability. The
development of appropriate statistical techniques is currently under way
at Tartu Astrophysical Observatory.
   Further photometric and polarimetric observations with substantial time
base can reveal more exactly the nature of slow variability of this
interesting object.


                         E. PAVLENKO
                 Crimean Astrophysical Observatory

                          J. PELT
                Tartu Astrophysical Observatory