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\IBVShead{5162}{29 August 2001}

\IBVStitle{GSC 8527-373: a Multimode Delta Scuti Star}

\IBVSauth{P\'ocs, M.D.$^1$; Rea, R.D.$^2$; Szeidl, B.$^1$}

\IBVSinst{Konkoly Observatory, H-1025 Budapest, P.O. Box 67, Hungary,
e-mail: pocs@konkoly.hu, szeidl@konkoly.hu}

\IBVSinst{Regent Lane Observatory, Nelson, New Zealand, e-mail: reamarsh@ihu.co.nz}

\SIMBADobj{GSC 8527-373}

\begintext

The variability of GSC 8527-373, an approx.\ 12.5-mag star was
discovered by Rea (2001) while monitoring the near-by CV star TW
Pictoris. On the basis of the period ($0\fday080$) and amplitude
($0\fmm2$) of the star's light variation the object was classified as a
probable $\delta$ Scuti star.  Since the variable has been thoroughly
observed, it prompted us to rediscuss the observations and to look for
further frequencies.

The original observations were made with an 0.35-m Schmidt--Cassegrain
telescope and a CCD detector without using any filter, thus no colour
information was obtained and no utilizable information was available
from other sources either for this object. On the other hand the lack
of filters may result in severe zero point shifts from night to night
because of the colour dependence of atmospheric transparency. The
observations have been obtained on 14 nights during two months:
December 6, 7, 9, 12, 13, 21, 26, 2000, January 3, 13, 17, 26, 29, 31
and February 1, 2001.

Before the analysis heliocentric corrections were applied to the ``raw"
data and then, in order to decrease the scatter, the observations were
binned in groups of three. Multifrequency analysis was performed with
the MUFRAN (MUltiFRequency ANalysis) program package (Koll\'ath,
1990).  MUFRAN is a collection of methods for period determination,
sine fitting for observational data and graphics routines for
visualization of the results.

The first, rather superficial frequency analysis clearly showed a high
peak at $f_1=12.5521$ c/d, the main frequency of the star. After
prewhitening with the frequencies $f_1$, $f_2=2f_1$ and $f_3=3f_1$ the
residual spectrum seemed to be very noisy. Our suspicion was that it
might be the result of the defectiveness of the data. Therefore the
data sets of different nights were carefully scrutinized and it turned
out that the scatter of the observations were excessively large on the
nights 12, 13 December, 2000 and 29 January, 2001. These observations
were left out of consideration in the final analysis.  (If we took into
account the less noisy data of these nights our final results did not
change.)

Fig.~1 shows the spectral window and Fig.~2 presents the Fourier
amplitude spectrum of the data of 11 nights. The frequency 12.5521 c/d
and its multiples are present, and after prewhitening with them
(Fig.~3), a further frequency $f_4=18.87660$ can be deduced. The
results of the least-squares solution with these frequencies are given
in Table~1. The residual is 0\fmm013 which seems to be slightly high
since the error of the binned observations is around 0\fmm005.

After removing the frequencies $f_1$, $f_2=2f_1$, $f_3=3f_1$ and $f_4$, the
remaining spectrum is shown in Fig.~4. The high peaks at the short
frequency end refer to serious zero point shifts from night to night.
Although real frequencies may exist on the short frequency ($f\leq 1$
c/d) domain (see e.g.\ Papar\'o et al.\ 1996), in the present case the
previous explanation seems to be valid. Probably other frequencies
($f_5=10.3658$ c/d, $a_5=0\fmm003$; $f_6=18.6727$ c/d, $a_6=0\fmm003$)
are also present, but the available observational material does not
allow further discussion and conclusion.

The asymmetric light-curve ($f_2=2f_1$ and $f_3=3f_1$ are also present) and
the low amplitude ratio $a_4/a_1=0.072$ make the object a very interesting
$\delta$ Scuti star. The high amplitude oscillation (with the frequency $f_1$)
may be identified as the fundamental radial mode, and the frequency $f_4$ (and
possible other frequencies) as non-radial mode(s).

According to its behaviour the star resembles the unique high-amplitude
$\delta$ Scuti star AN Lyncis (Rodr\'\i guez et al.\ 1997) in many respects
(e.g.\ the amplitude ratio of the non-radial and radial oscillation or the
frequency distribution).

The star is certainly a good target for further investigation.

\begin{table}[h]
\caption{Least-squares solution}
\begin{center}
\begin{tabular}{cccc}
\hline
&  frequency (${\rm d}^{-1}$) & amplitude (mag) & phase (rad) \\
\hline
$f_1$      & 12.55213 & 0.069 & 0.56 \\
$f_2=2f_1$ & 25.10426 & 0.010 & 2.71 \\
$f_3=3f_1$ & 37.65639 & 0.002 & 0.76 \\
$f_4$      & 18.87660 & 0.005 & 5.63 \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10cm}{5162-f1.ps}{Spectral window}

\IBVSfig{10cm}{5162-f2.ps}{Amplitude spectrum of the binned observations of
11 nights}

\IBVSfig{10cm}{5162-f3.ps}{Amplitude spectrum of the binned observations of 11
nights after removing the frequencies $f_1$, $f_2=2f_1$, $f_3=3f_1$}

\IBVSfig{10cm}{5162-f4.ps}{Amplitude spectrum after removing the frequencies
$f_1$, $f_2=2f_1$, $f_3=3f_1$ and $f_4$}

{\it Acknowledgement.} This work was supported by the Hungarian Research
Grant OTKA T-030955.

\references

Koll\'ath, Z., 1990, {\it Occasional Technical Notes, Konkoly Obs.}, No.\ 1

Papar\'o, M., Sterken, C., Spoon, H.W.W., \& Birch, P.V., 1996, {\it A\&A}, {\bf 315}, 400

Rea, R.D., 2001, {\it IBVS}, No.\ 5064

Rodr\'\i guez, E., Gonz\'alez-Bedolla, S.F., Rolland, A., Costa, V.,
L\'opez-Gonz\'alez, M.J., \& L\'opez de Coca, P., 1997, {\it A\&A}, {\bf 328}, 235

\endreferences

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