COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1848

                                   Konkoly Observatory
                                   Budapest
                                   1980 September 26

                                   HU ISSN 0374-0676



         ON THE VARIABILITY OF CENTRAL STARS OF
               TWO PLANETARY NEBULAE




  There are several indications on the existence of rapid
small-scale oscillations of the central stars of planetary
nebulae (see Kohoutek, 1966; Lawrence et al., 1967; Stothers,
1977). Amplitudes of oscillations lie in the range of thousandths 
and hundredths of stellar magnitude on time scale extending 
from minutes to several weeks. Photoelectric observations 
of two planetary nebulae (NGC 7009, m_pg=8.5 and NGC 7662 m_pg=
9.0) were carried out during three nights in September, 1979 in
order to detect and study their microvariability. The nebula
diameters are less than one minute of arc. Observations were
made at the Abastumani Astrophysical Observatory on the 48-cm
telescope with 50" diaphragm by the photon counting method. Integrations 
of 10sec in B filter were used. In order to control
the atmospheric properties of the night the normal star HD 21479
(A2V,m_v=7.28) was observed after the nebulae. The data are given
in Table I.
  Table I
Object          Date      Number of           Mean-square
                          observations          amplitude
                                                 (mag.)
NGC 7009    14- 15.9.1979      740              0.0048
NGC 7662    20- 21.9.1979      916              0.0046
            21- 22.9.1979      896              0.0036
HD 21479    21- 22.9.1979       95              0.0024

    Figs. 1 and 2 show the autocorrelation functions (ACF)
and the power-spectra of brightness variations. The results are
the following:
     1) Both nebulae display the microvariability with amplitudes 
of 0.002-0.003 mag and periods 82 min (NGC 7009) and


                         [FIGURE 1]
                         [FIGURE 2]

60-64 min (NGC 7662);
    2) The frequency spectrum shows no significant variations
from night to night, amplitudes in this case may change noticeably 
(Fig. 2, NGC 7662).
    3) Comparison of ACF and power-spectra of NGC 7662 with
those of the control star in the same night (in Fig.2 depicted
by circles) testifies to the physical variability of the nebulae
in brightness rather than variability due to fluctuations in
atmospheric transmission.
    The authors suppose that the microvariability of the planetary 
nebulae indicate on the activity of their hot central stars,
concentrated in the UV region.






                      B.E. ZHILJAEV

                      Main Astronomical Observatory
                      of the Ukrainian Academy of
                      Sciences, Kiev, USSR


                      A.G. TOTOCHAVA

                      Abastumani Astrophysical
                      Observatory of the Georgian
                      Academy of Sciences,
                      Abastumani, USSR




References:
Kohoutek, L., 1966, Bull.Astron.Inst.Czechoslovakia, 17, 318 [BIBCODE 1966BAICz..17..318K ]
Lawrence, G.M., Ostriker, J.P., Hesser, J.E., 1967, Ap.J.148,L161 [BIBCODE 1967ApJ...148L.161L ]
Stothers, R., 1977, Ap.J., 213, 791 [BIBCODE 1977ApJ...213..791S ]