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\begin{document}
\IBVShead{4265}{14 November 1995}
\title
{HS2324+3944: a new H--rich pulsating PG\,1159 White Dwarf}

\begintext

With an effective temperature of (130\,000 $\pm$ 10\,000) K, and a
surface gravity corresponding to log $g$ = 6.2 $\pm$ 0.2,
HS\,2324+3944 is a peculiar PG\,1159 star: it is the only star of this class,
not surrounded by a nebula, showing H Balmer absorptions in its spectrum
(Dreizler et al., 1995).
If the pulsation mechanism based on the C/O cyclic ionization, proposed by
Starrfield et al. (1984), is at work, the high H abundance found in the
atmosphere (He/H = 0.5 by number) should drop to zero very quickly in the
driving regions.
Such a strong decrease of hydrogen looks quite unlikely; for this reason
the presence of pulsations in HS\,2324+3944 seems to be a very interesting
phenomenon.

I observed HS\,2324+3944 with the 2-head photoelectric photometer of the 1.5 m
Loiano telescope (Bologna Astronomical Observatory) on October 19 and 20, 1995,
with no moon.
The tubes used, two EMI 9784 QB, have a maximum sensitivity in the B band.
Both observations were done without filter, with an integration time of 2 s.
The original light curves are shown in Figure 1 and 2, whereas in Figure 3 and 4
the magnitudes corrected for extinction are presented.
The comparison stars of the two observations are different.
In the first observation the comparison star (RA=23\fh 27\fmm 33\fs, D=+39\degr 
48\farcm 9 (1950.0)) is about 0.5 mag fainter than
HS\,2324+39 whose V-magnitude is about 14.8.
The sharp light increment between BJD 0.317 and 0.410 in October 19 is not due
to any instrumental failure or astronomical reason.
This effect disappears almost completely taking the difference of magnitude
(Figure 3) (not completely because the diaphragms of the 2 channels of the
photometer are different).
In the second observation the comparison star (RA=23\fh 27\fmm 35\fs ($\pm$18\fs); D=+39\degr 43\farcm 6 ($\pm$3\farcm 5) (1950.0)) is about 2 magnitudes brighter
than HS\,2324+39.
For this reason there is almost no difference between channel 1 and (1--2) in
Figure 4. Near BJD 0.41 (October 20) the comparison was close to the border
of the aperture.
In both light curves we can see several arc features with a mean period
of about half an hour.
In Figure 5 the data distribution for both nights is shown, folding the
two light curves with a period of 35 min.
In the power spectra of the two nights the amplitudes of the 35 min pulsation
are 9 mmag (19/10/95) and 15 mmag (20/10/95).
Apart from the 35 min periodicity, there are not other peaks with amplitude
higher than 5 mmag in both power spectra.
Probably more data are required to find other possible pulsation frequencies.
In any case further and more detailed analysis of these data will be performed.
\vfill \eject

\centerline{2}
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\centerline{\epsffile{4265-f1.ps}}
\centerline{Figure 1. Counts light curve of HS\,2324+3944 in October 19, 1995.}

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\centerline{Figure 2. Counts light curve of HS\,2324+3944 in October 20, 1995.}
\vfill \eject

\centerline{3}
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\centerline{\epsffile{4265-f3.ps}}
\begin{center}
Figure 3. Magnitude light curve of HS\,2324+3944 in October 19, 1995.
The spurious light increment in channel 1 and channel 2, from about BJD 0.317
to 0.410, disappears almost completely considering the difference of magnitude.
\end{center}

\epsfysize=95mm
\centerline{\epsffile{4265-f4.ps}}
\begin{center}
Figure 4. Magnitude light curve of HS\,2324+3944 in October 20, 1995.
The comparison star has been recentered in the aperture near BJD 0.413.
\end{center}
\vfill \eject

\centerline{4}
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\centerline{\epsffile{4265-f5.ps}}
\begin{center}
Figure 5. Distribution of the data of both nights, folded with a period
of 35 min.
\end{center}

\vskip 1.0truecm
I am pleased to acknowledge Klaus Werner, who suggested to observe this star
and provided me the finding chart.


\vskip 1cm

\beginauthors
\authorsrightcol
Roberto SILVOTTI
Dipartimento di Astronomia
Universit\`a di Bologna, Italia
\endauthors

\references

Dreizler, S., Werner, K., Heber, U., Engels, D., 1995, {\it A\&A} in press

Starrfield, S., Cox, A.N., Kidman, R.B., Pesnell, W.D., 1984, {\it ApJ}, {\bf 281}, 800


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