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\centerline{\ft PHOTOMETRIC VARIATIONS OF THE CENTRAL STAR OF M\,1-77}
\centerline{\ft AND SUSPECTED VARIABILITY OF THE CENTRAL STAR OF VV\,3-5}
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In recent years, more and more ``cool'' central stars of planetary nebulae were
discovered to show
irregular or, at best, semi-regular photometric variations. Currently, 10 such
objects were reported in the literature, namely the central stars of NGC\,40,
NGC\,6543, NGC\,6826, IC\,418, IC\,2149, IC\,3568, IC\,4593, Hu 2-1, He 2-131 and 
He\,2-138 (Bond \& Ciardullo, 1991, Hutton \& M\'endez, 1993). Most of these 
objects do
not appear to be binaries, and their variability is attributed to either wind
variations or pulsations by the different authors.

In order to examine the reason for the variability of these stars, we have
started a photometric search for related objects enabling us
to learn more about possible group properties. Here we report the
discovery of light variations in the central star of M\,1-77 and suspected
variability of the central star of VV\,3-5.

Our observations were carried out in August 1995, with the Texas two-channel
photometer attached
to the 0.9~m telescope at McDonald Observatory, employing only channel 1 to
acquire differential photometry. All our target stars have small nebulae,
allowing us to include the whole nebula in the aperture. This prevents variable
influence of the nebular background.

We chose two comparison stars (C1 and C2) for each object, measuring them
together with the planetary nebula (PN) in the order: C1--PN--C2--C1--PN--C2--... The comparison stars, typically of 9--10 mag, were integrated for about 60
seconds, the planetary nebula about 120--180 seconds, depending on its 
magnitude. A Johnson V filter was mainly used, in order to minimize the nebular
contribution to the data without losing too many photons from the central star.

All data were corrected for sky background and for extinction. No dead-time
correction was applied, since the count rates were lower than 15\,000 counts
per second. The relative zeropoints of the measurements of the
different stars were calculated for the first night and subtracted from the
data except for a small offset for plotting purposes. These relative zeropoints
were applied for all other nights of observation. The resulting light curves
were analysed for variability.

Let us first describe our findings for M\,1-77 in detail; its light curve can be
found in Figure 1. M\,1-77 appeared to be variable already in the first night we
observed it. However, since the light modulation was very well correlated with
air mass, we first suspected that we could be confronted with an artifact
caused by strong nebular emission, i.e. we could be measuring the central star
at an effective wavelength different from that of the comparison stars. 

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Figure 1: Differential time-series photometry of M\,1-77 (PN),
SAO\,50704 (C1) and SAO\,50708 (C2). M\,1-77 is clearly variable.
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Figure 2: Differential time-series photometry of VV\,3-5 (PN), SAO\,161649 (C1) and SAO\,187052
(C2). We suspect that VV\,3-5 is variable.
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Therefore, we re-observed the star with both the V and the y filter three nights
later. The y filter is almost a continuum filter for planetary nebulae, and
since M 1-77 is bright, we did not have to increase the integration time too
much. In this night, the star did not appear to be variable (middle panel of
Figure 1) on time scales of hours. However, it was constant in {\it both} filters,
convincing us that the variations in the first night were intrinsic. Note that
the mean magnitude of the star has changed from the first night to the second
night. We again observed the star two nights later, finding that it was even
dimmer than on the second night. Thus, we consider its variability to be well
established. Moreover, M\,1-77 behaves very similarly to the best investigated
``cool'' variable central star, HD\,35914, the central star of IC\,418 (M\'endez et al., 1986).

Our observations of VV\,3-5 are somewhat harder to interpret. First, we note
that the star has a companion about 1.5 magnitudes brighter in approximately 20
arcseconds distance. This companion was always carefully excluded from the
aperture. Since telescope tracking was excellent, VV\,3-5 or the companion
did not move towards the edge of the aperture during the integration.
However, the photometric quality of the nights during which
VV\,3-5 was observed was not as good as those we spent on M\,1-77. Our light
curves are displayed in Figure 2.


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In both nights, the light curves of VV\,3-5 did not follow those of the 
comparison stars closely. Moreover, the zeropoints were also not the same in
the different nights. On the other hand, the magnitude difference of the
comparison stars also changed slightly. This can be attributed to bad photometric
quality or to intrinsic variability of at least one of the comparison stars.
The latter would be somewhat surprising, since both comparison stars have
spectral types of B8.

Consequently, we can only suspect that VV\,3-5 is intrinsically variable.
We strongly suggest further observations of this star, preferably with CCDs
because of the companion mentioned above.

The first part of our survey for photometric variations among ``cool'' central
stars of planetary nebulae increased the number of variables to 11, maybe 12.
The relatively large number of variables suggests that light modulation might
be a rather common phenomenon for these stars. Therefore, the discovery of 
further related
objects is rather of statistical interest, but should not be considered as a
great surprise.

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{\it Acknowledgements:} This research was partially supported by the Austrian
Fonds zur F\"orderung der wissenschaftlichen Forschung under grant S-7309.

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% Call for macros \authors - \authorsrightcol - \endauthors
\beginauthors
G. HANDLER $^{\star}$
Institut f\"ur Astronomie
T\"urkenschanzstra\ss e 17
A-1180 Wien, Austria
E-mail: HANDLER@ASTRO.AST.UNIVIE.AC.AT
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$^{\star}$ Guest observer
McDonald Observatory,
The University of Texas at Austin
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% Call for macro \references
\references

Bond, H. E., Ciardullo, R., 1990, {\it ASP Conf. Ser.}, 11, 529, 
in {\it Confrontation between stellar pulsation and evolution}, eds. C.~Cacciari \& G.~Clementini
%Original: Bond, H. E., Ciardullo, R., 1991
%Corrected: Wed Jun  1 11:22:45 MEST 2005

Hutton, R. G., M\'endez, R. H., 1993, {\it A\&A}, {\bf 267}, L8

M\'endez, R. H., Forte, J. C., L\'opez, R. H., 1986, {\it Rev. Mex. Astron. Astrofis.}, {\bf 13}, 119

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