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\IBVShead{5467}{17 October 2003}

\IBVStitletl{A CCD Photometric Search for Pulsations in SZ~H\lowercase{er}}

\IBVSauth{SZ\'EKELY, P.$^{1,2,3}$}

\IBVSinst{Department of Experimental Physics and Astronomical Observatory,
University of Szeged, D\'om t\'er 9, Szeged, Hungary, H-6720}
\IBVSinst{School of Physics, University of Sydney 2006, NSW Australia}
\IBVSinst{Guest observer at Konkoly Observatory}
\IBVSinsto{e-mail: pierre@physx.u-szeged.hu}

\SIMBADobj{SZ Her}
\IBVStyp{ EA }

\IBVSabs{New VRI photometry of the Algol-type eclipsing binary SZ Her }
\IBVSabs{is presented. The data were acquired on 9 nights in 2002 yielding}
\IBVSabs{to four new epochs of primary minimum. After subtracting}
\IBVSabs{the mean eclipsing variation, the residual data were analysed}
\IBVSabs{in search for oscillations. We found, however, only upper limits}
\IBVSabs{in the frequency spectra.}

\begintext

In a recent comprehensive catalogue of $\delta$ Scuti type pulsating stars
there are 86 objects belonging to binary or multiple stellar systems
(Rodr\'\i guez et al., 2000).  Of those, only 9 stars are components of eclipsing
binaries (Rodr\'\i guez \& Breger, 2001). Recent surveys revealed a few more
eclipsing binaries with pulsating components (e.g. Kim et al., 2003). These
stars are desired compound for asteroseismology in order to identify pulsation
modes through determination of fundamental physical parameters. Another
interesting possibility is the study of tidal effects on oscillation (Willems
\& Aerts, 2002). 

Inspired by this importance, we started a photometric survey of Algol-type
eclipsing binaries for pulsating components. Since $\delta$ Scuti stars are
main-sequence or slightly evolved A--F type stars in the lower parts of the
instability strip, our targets were selected according to the spectral type of
the components. Basic data were extracted primarily from the Hipparcos database
(ESA 1997). In this paper we present the results obtained for SZ~Herculis 
(V=9\fmm92, $\Delta$V=1\fmm75, mean spectral type F0V). 

Photometric monitoring 
of this star began early  (Dugan, 1923), the first visual minima are dated as
back as 1902. According to light curve analysis of Giuricin \& Mardirossian
(1981), the system is semi-detached with a Roche-lobe filling subgiant
secondary star. Cyclic O$-$C variations were noted as well (Mallama
1980, Zavala, 2002), although to our knowledge, there has not been any detailed
period change study of the star.  

We carried out standard Johnson--Cousins VRI filtered CCD observations on 9
nights in June and July of 2002. In order to obtain good time-resolution, the
observations were single-filtered during the given night. The achieved
photometric accuracy varies between 0.01--0.05 mag depending on the weather
conditions. The bulk of the data was acquired with the 0.4m
Cassegrain-telescope of the Szeged Observatory. This telescope was used with a
cooled SBIG ST--9E CCD camera (512$\times$512 20$\mu$m sized, 2$\times$2 binned
pixels, field of view was $6\arcm \times6\arcm$). The exposure times were 30
second. One night of data was obtained with the  60/90/180 cm Schmidt-telescope
of the Konkoly Observatory, equipped with a Photometrics AT200 CCD camera
(1536$\times$1024 KAF 1600 MCII coated CCD chip). The projected area is
29$\arcm \times$18$\arcm$ which corresponds to an angular resolution of
1\farcs1/pixel. Applying only a few second exposures -- typically 5 seconds --
we were able to accomplish relatively fast photometry thus the resulting light
curve consists of more than 1600 data points. The complete dataset contains of
3 V-filtered, 2 R-filtered and 4 I-filtered nights with 5481 individual points
covering 55.4 hours (see Table 1). The dataset is available at the IBVS
website as {\tt 5467-t2.txt}.

\IBVSedata{5467-t2.txt}
%%\IBVSdataKey{5467-t2.txt}{SZ Her}{photometry}

\begin{table}
\caption{Log of observations and times of minimum.}
{\small
\begin{center}
\begin{tabular}{lclclll}
\hline
\hline
Obs. Date & Length [h] & Telescope & Filter & HJD$_{\rm min}$ &Type & O$-$C (day)\\ 
\hline
\hline
20.06.2002 & 5.76 & 0.4m Cass. & V & 2452446.3795(2) &primary & 0.0034\\
21.06.2002 & 6.48 & 0.4m Cass. & V & & & \\
24.06.2002 & 6.24 & 0.4m Cass. & V & 2452450.4706(2) &primary & 0.0040\\
05.07.2002 & 6.48 & 0.4m Cass. & R & 2452461.5152(10) &secondary & 0.0043\\
08.07.2002 & 5.28 & 0.4m Cass. & R & 2452464.3777(3) &primary & 0.0035\\
10.07.2002 & 6.24 & 0.4m Cass. & I & 2452466.4207(3) &secondary & 0.0013\\
11.07.2002 & 6.96 & 0.4m Cass. & I & & & \\
15.07.2002 & 7.20 & 0.4m Cass. & I & & & \\
21.07.2002 & 4.80 & 0.6m Schmidt & I &2452477.4705(1) &primary & 0.0068\\
\hline
\hline
\end{tabular}
\end{center}
}
\end{table}

For differential photometry we used the nearby stars GSC 2610--1214
(V=11\fmm82) and GSC 2610--1417 (V=13\fmm11) as comparison and check stars,
respectively. We were able to determine six epochs of minimum, four primaries
and two secondaries by fitting low-order polynomials to selected parts of the
light curves. We list the minima in Table 1. The O$-$C data were calculated 
using the ephemeris $HJD_{\rm min}=2434987.3959+0.818095693 E$, while the 
light curve was phased with $HJD_{\rm min}=2452446.3795+0.8180644 E$. 
The phased
instrumental light curves in V,R and I are shown in Fig.\ 1. The light curve is
characterized by large eclipse depths ($\Delta$V=1\fmm75,
$\Delta$R=1\fmm62, $\Delta$I=1\fmm5) and slight reflection effect or
ellipsoidal variations of about 0\fmm1.

\IBVSfig{8cm}{5467-f1.eps}{The VRI phase diagrams of SZ~Her. The curves are shifted vertically for clarity.}
%%\IBVSfigKey{5467-f1.eps}{SZ Her}{light curve}

At first sight the light curves did not show discernible light variations due
to short period oscillation of any of the components. To assert more precisely
the lack of short-term light variations, we performed frequency analysis of the
individual light curves. For this, we calculated the binned phase
diagram (the bin size was 0.01), which was transformed back to the 
time domain over the time span of observations. After subtracting 
these smooth data from the original observations, the residuals 
showed the deviations from the mean.

For this analyses, we used {\tt Period98} written by Sperl (1998). The resulting
Fourier spectra with the window functions are plotted in Fig.\ 2. We can
summarize the results as follows. We could not identify any characteristic
frequency produced by coherent oscillation in the residual light curves with
amplitudes larger than 0\fmm004 in V and R and 0\fmm007 in I, respectively.
We made simulations to estimate possible decrease of the Fourier-amplitude  by
the observational errors. Artificial datasets were created consisting  of a
low-amplitude single sine wave and different scatter levels  mimicking our
measurement errors. We found that the calculated amplitudes  agreed very well
with the initial value (within a few percent), so that  our amplitude limits
are only slightly affected by the noise. Thus we can certainly assert there is
no $\delta$ Scuti-type pulsator in SZ~Her with larger amplitudes of pulsation
than these upper limits. 

\IBVSfig{10cm}{5467-f2.eps}{Frequency spectra of the
residual light curves. The small inserts show the window functions.}
%%\IBVSfigKey{5467-f2.eps}{SZ Her}{periodogram}

In addition to this, we have checked the star's recent period change. The O$-$C
diagram (Kreiner 2001)\footnote{Available electronically at {\tt 
http://www.as.wsp.krakow.pl/o-c/data/getdata.php3?SZ\%20her}} plotted in 
Fig.\ 3 shows noticeable variations. Assuming that the changes are periodic, they can
be approximated by a sine wave with 0\fday015 amplitude and 66 year period.

\IBVSfig{7cm}{5467-f3.eps}{The O$-$C diagram of SZ~Her.}
%%\IBVSfigKey{5467-f3.eps}{SZ Her}{periodogram}

A possible interpretation is the light-time effect produced by the binary
system orbiting around a third component. Assuming these objects are orbiting
approximately in the same plane as the components of the binary system do
(which is the usual assumption for hierarchic systems in celestial mechanics,
and, consequently, the inclination is close to 90\deg), applying Kepler's 
third law, we can estimate the mass function of the third component (Zhou \& Fu,
1998): $(\Delta tc)^3/P_{\rm orb}^2=f(M_{3_{\rm min}})$, where $\Delta$t is the
semi-amplitude of the O$-$C diagram, c is the speed of light and $P_{\rm orb}$
is the hypothetical orbital period, i.e 66 years. With parameters described above
we can get $f(M_{3_{\rm min}})=0.004M_{\odot}$. According to this, the minimum
mass of the accompanying object roughly equals four Jupiters. One possibility
is that the object might be a giant planet or a brown dwarf star in case of
smaller inclinations.

\medskip

{\footnotesize
\noindent This work has been supported by the Hungarian Ministry of Education (FKFP Grant 0010/2001), Hungarian Research Council (OTKA Grant F043203), PRCH Student Science Foundation and the Australian Research Council. The author wishes to thank the assistance with observing of Bal\'azs Cs\'ak and lots of advices and help of L.L. Kiss.
This research has made use of the SIMBAD database, operated at CDS-Strasbourg,
France. The NASA ADS Abstract Service was used to retrieve data and references.}

\vspace*{-0.5cm}

\references

Dugan, R. S., 1923, {\it ApJ}, {\bf 58}, 164

ESA, 1997, {\it The Hipparcos and Tycho Catalogues}, ESA-SP 1200

Giuricin, G., Mardirossian, F., 1981, {\it A\&AS}, {\bf 45}, 85

Kim, S.-L., Lee, J. W., Kwon, S.-G. et al., 2003, {\it A\&A}, {\bf 405}, 231

Kreiner, J. M., Kim, C.-H., Nha, I.-S., 2001, {\it An Atlas of O$-$C Diagrams of Eclipsing Binary Stars}, \\
Wydawnictwo Naukowe Akademii Pedagogicznej

Mallama, A. D., 1980, {\it ApJS}, {\bf 44}, 241

Rodr\'\i guez, E., Breger, M., 2001, {\it A\&A}, {\bf 366}, 178

Rodr\'\i guez, E., L\'opez-Gonz\'alez, M. J., L\'opez de Coca, P., 2000, {\it A\&AS}, {\bf 144}, 469

Sperl, M., 1998, {\it Comm. Astr. Seis.}, {\bf 111}

Willems, B., Aerts, C., 2002, {\it A\&A}, {\bf 384}, 441

Zavala, R. T., McNamara, B. J., Harrison, T. E. et al., 2002, {\it AJ}, {\bf 123}, 450

Zhou, A.-Y., Fu, J.-N., 1998, {\it Delta Scuti Star Newsletter}, {\bf 12}, 28


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