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\centerline{\ft THREE NEW VARIABLE PLANETARY NEBULA CENTRAL STARS:}
\centerline{\ft M\,2-54, M\,4-18 AND NGC\,2392}
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We are presently engaged in a search for variable central stars of young
planetary nebulae in order to shed more light on the cause
of the variability of these stars. Previously, this has increased the number
of known variables to 12 (Handler, 1995). In this note we report the discovery
of three further related objects.

Our observations were carried out in August to December 1995 with the Texas 
two-channel photometer attached to the 0.9m and 2.1m telescopes at McDonald 
Observatory, respectively. We employed only channel 1 to acquire differential
photometry. To 
prevent variable influence of the nebular background caused by variable
seeing or poor guiding, we always included the whole nebula in the aperture.

We measured two comparison stars (C1 and C2) together with the
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--240 seconds, depending on its 
magnitude. In order to minimize the nebular contribution to the data, Johnson
$V$ or Str\"omgren $y$ filters were used.

All data were corrected for sky background and for extinction. No dead-time
correction was applied. 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. To allow the reader to judge the quality of the
data, we did not compensate for variations in sky transparency or for possible
tube drifts.

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\noindent
{\bf{M\,2-54:}} All the observations of this object were acquired with the 0.9m
telescope through a Johnson $V$ filter. We first measured the star on August 30,
1995. The light curve
(Figure 1, left panel) was not of very good quality. However, a drift in the
magnitudes of M\,2-54 was visible, leading us to suspect that the central
star could be variable. Consequently, we obtained a second run on October 23,
1995 under much better photometric conditions. Fortunately, the star also
co-operated and became fainter by almost 0.2 magnitudes in 6 hours (Figure 1,
right panel).


Needless to say, we conclude that M\,2-54 is variable. However, from the
present data we cannot infer the timescale of the light modulations. More
extensive observations, preferably from at least two sites, are necessary.
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Figure 1: Light curves of M\,2-54 (crosses) relative to the comparison stars
SAO\,34899 (dots) and SAO\,34900 (diamonds)
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Figure 2: Time-series photometry of M\,4-18 (crosses) relative to the comparison
stars BD+60\degr 806 (dots) and SAO\,13101 (diamonds)
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Figure 3: Light curve of NGC\,2392 (crosses) relative to the comparison
stars BD+21\degr 1613 (dots) and SAO\,79446 (diamonds)

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We are careful to note that the gap in the August data was not caused by bad
photometric conditions, but by a temporal failure of telescope tracking. The
trend in the comparison star data taken in October is due to tube drift and is
not caused by a wrong choice of the extinction coefficient.

\noindent
{\bf{M\,4-18:}} We observed this object with the McDonald Observatory 2.1m 
telescope in the night of December 12, 1995 through a Johnson $V$ filter 
(Figure 2). The central star brightened by about 0.03 mag during the
7 hours of observation. Thus, it is
the lowest amplitude variable we discovered so far. Due to the good photometric
conditions, under which the measurements were acquired, we consider the
variability of M\,4-18 to be well established. 


Similarly to our conclusion for M\,2-54, we recommend more observations of 
M\,4-18 to unravel the timescale of its light modulations. However, due to
the faintness of the star ($V \approx 13.9$), CCD observations or a large
telescope are required.

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\noindent
{\bf{NGC\,2392:}} We measured this planetary nebula on December~30, 1995 with
the 0.9m telescope. Since the object consists of both a bright central star 
(HD\,59088) and a large nebula (the famous Eskimo Nebula), we acquired our data
through the Str\"omgren $y$ filter. The resulting light curve is shown in Figure 3.



The timescale of the light modulations seems to be near 4--5 hours. However,
since the variability of this kind of central stars is not strictly periodic
(e.g.~Handler et al., 1996), that value can be in error. NGC\,2392 would be an
object very well suited for both spectroscopic and photometric multisite
observations.

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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 and
by the Austrian Zentrum f\"ur Auslandsstudien.

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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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\references

Handler, G., 1995, {\it IBVS}, No.\,4244

Handler, G., M\'endez, R. H., Medupe, R., et al., 1996, {\it A\&A}, in preparation

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