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\IBVShead{5205}{21 November 2001}

\IBVStitle{GSC 2118-00297: a New Double-Mode {\boldmath $\delta$} Scuti Variable}

\IBVSauth{Van Cauteren, P.$^{1,2}$; Wils, P.$^2$}

\IBVSinst{Beersel Hills Observatory, Laarheidestraat 166, B-1650 Beersel, Belgium,
email: Paul.VanCauteren@pi.be}
\IBVSinst{Vereniging Voor Sterrenkunde, Belgium, email: Patrick.Wils@cronos.be}

\SIMBADobj{ROTSE1 J182943.22+280955.2}
\IBVSkey{Delta Scuti stars - photometry}
\IBVSabs{Unfiltered CCD photometric data show that GSC 2118-00297 is a}
\IBVSabs{double-mode Delta Scuti variable with a fundamental period of 0.145559 days and}
\IBVSabs{a period ratio of 0.766.}

\begintext

The star GSC 2118-00297 (= ROTSE1 J182943.22+280955.2; $\alpha_{2000}= 18\hr29\mm43\sec$;
$\delta_{2000}= +28\deg09\farcm9$) was announced by the ROTSE1 (Robotic Optical Transient
Search Experiment 1) survey (Akerlof et al., 2000) to be a $\delta$ Scuti variable
with a period of 0\fday170407 in the approximate magnitude range 12.6-12.9.

The star was observed at Beersel Hills Observatory on six nights between August and
October 2001.  A total of 760 data points were obtained during 15.4 hours
of photometry. The instrument used was a 0.40-m telescope, equipped with a 
ST7E CCD camera. No filter was used. The exposure times varied between 50 and 90 seconds.
The images were reduced with the aperture photometry procedure of the Mira
AP software package\IBVSfm.\IBVSfoot{The Mira AP software is produced by 
Axiom Research Inc.}

The brightness of the variable was measured with respect to GSC 2118-00221, having
a colour ($\Delta (B - V)$= 0.19) closest to the variable, from among neighbouring stars.
GSC 2118-00299 ($\Delta (B - V)$= 0.56) and GSC 2118-00292 ($\Delta (B - V)$= 0.26) served as check 
stars. The instrumental values of $\Delta (B - V)$
were determined from images in B and V light, using a filterset following Bessel's specifications.
The nightly standard deviation of the differences in unfiltered magnitudes between the comparison and
the first check star averaged 0\fmm012, and 0\fmm023 for the second check star.  The check stars 
respectively have an instrumental magnitude of $-$0.90 and 0.64 with respect to the comparison star. 

Using the Fourier analysis program Period98 (Sperl, 1998), the dominant frequency in the 
observations turned out to be 6.86961 c/d (see the Fourier periodogram in fig.~1), a one-day alias 
of the period given by Akerlof et. al. (2000).  The peak to peak amplitude of this frequency is 
0\fmm30.  Frequencies below 3 c/d in the power spectrum were not considered as the density of 
observations at these frequencies
is too low (the longest observing run being 4.2 hours on a total observation interval of 59 days).

After prewhitening for the fundamental frequency and its first harmonic (13.73923 c/d 
with an amplitude of 0\fmm06), a second frequency of 8.96848 c/d was found (see fig.~2),  
making this star a new double-mode $\delta$ Scuti star.  The ratio of the first overtone to
the fundamental period is 0.766, on the low end of the narrow observed range for this ratio
in other double-mode $\delta$ Scuti stars (Petersen and Christensen-Dalsgaard, 1996).
The amplitude of the second frequency is 0\fmm03, one tenth of the amplitude of the 
fundamental frequency, but well above the error bar on individual observations.  These will be on 
the order of 0\fmm012 as GSC 2118-00297 is of similar brightness as the first check star.

\newpage

\vspace*{-1.0cm}

\IBVSfig{6.1cm}{5205-f1.ps}{Fourier periodogram for GSC 2118-00297.}

\vspace*{-1.0cm}

\IBVSfig{6.1cm}{5205-f2.ps}{Fourier periodogram for GSC 2118-00297 after prewhitening for the fundamental period.}


The following times of maxima have been determined (O-C values are listed with respect
to the ephemeris derived below):
\begin{center}
\begin{tabular}{rcrc}
\hline
JD Hel. & E & O-C [d] & Observer \\
\hline
2451241.575 & -6217 & 0.000 & ROTSE1 \\
2452146.518 & 0 & 0.001 & PVC \\
2452175.334 & 198 & -0.003 & PVC \\
2452205.324 & 404 & 0.002 & PVC \\
\hline
\end{tabular}
\end{center}

The first maximum in the list is obtained from a phase diagram of the ROTSE1 data (available 
through http://www.umich.edu/\~{}rotse).  From these maxima, the following ephemeris was derived:
$$\begin{array}{r@{}r@{}l}
{\rm Max. = HJD}~2452146.516 & {} + 0\fday1455591 & {} \times E. \\
{} \pm 0.003 & {} \pm 0.0000014 \\
\end{array}$$

Fig.~3 shows the phased light curve from our data, folded with the fundamental period, and Fig.~4 the
phase diagram of the first overtone (after prewhitening for the fundamental period and its first harmonic).


{\bf Acknowledgements:} We thank Dr.~Patricia Lampens, Koninklijke
Sterrenwacht van Belgi\"e, for helpful comments. P. Van Cauteren is
grateful to the Royal Observatory of Belgium for putting at his disposal material
acquired by project G.0265.97 of the Fund for Scientific Research (FWO) - Flanders (Belgium).


\references

Akerlof, C., Amrose, S., Balsano, R., Bloch, J., Casperson, D., Fletcher, 
S., Gisler, G., Hills, J., Kehoe, R., Lee, B., Marshall, S., McKay, T., Pawl, 
A., Schaefer, J., Szymanski, J., Wren, J., 2000, {\it AJ}, {\bf 119}, 1901 

Petersen, J.O, and Christensen-Dalsgaard, J., 1996, {\it A\&A}, {\bf 312}, 463

Sperl, M., 1998, Manual for Period98 (V1.0.4). A period search-program for Windows and Unix, (http://dsn.astro.univie.ac.at/\~{}period98)

\endreferences

\vspace*{-0.2cm}

\IBVSfig{6.1cm}{5205-f3.ps}{Phase diagram for the fundamental period of GSC 2118-00297.}

\vspace*{-0.2cm}

\IBVSfig{6.1cm}{5205-f4.ps}{Phase diagram for the secondary period of GSC 2118-00297.}

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