\documentstyle[twoside]{article}

\input ibvs2.sty
\input ibvst.sty
\input epsf.tex

\begin{document}
\IBVShead{5280}{22 May 2002}

\IBVStitletl{V357\,H\lowercase{er}: Another W\,UM\lowercase{a}-Type Eclipsing Binary}{ Misclassified as HADS}

\IBVSauth{BRANICKI, A.$^1$; PIGULSKI, A.$^2$}


\IBVSinst{University in Bia{\l }ystok, Institute of Experimental Physics,
Lipowa 41, 15-424 Bia{\l }ystok, Poland, \\
e-mail: branicki@alpha.uwb.edu.pl}
\IBVSinst{Wroc{\l }aw University Observatory, Kopernika 11, 51-622
Wroc{\l }aw, Poland, e-mail: pigulski@astro.uni.wroc.pl}


\SIMBADobj{V357 Her}
\IBVSkey{CCD photometry}
\IBVSkey{Eclipsing binaries}

\IBVSabs{Using new CCD photometry of V357 Her we show that the star}
\IBVSabs{is misclassified: it is not a high-amplitude delta Scuti}
\IBVSabs{star as previously thought, but a W UMa-type eclipsing}
\IBVSabs{binary. The parameters of the system are derived.}



\begintext

The variable V357~Her (preliminary designation S\,4350~Her) was
discovered photographically by G\"otz and Wenzel (1956).  These authors
derived five times of maximum light and a period of 0.139725~d.
Considering the period and the shape of the light curve, the star was
classified as an RR~Lyrae-type variable, reclassified later as a
high-amplitude $\delta$ Scuti (HADS) star.

Not only for V357~Her, but also for some other stars classified as
HADS, either there are no observations since the time of discovery or the
observations are very scarce.  With CCD cameras, accuracy of 0.01~mag is easily
achieved, so the true nature of these stars can be verified even with
a small telescope.  This prompted us to carry out CCD observations of
some poorly studied stars classified as HADS.  The two stars we
observed a few years ago, UW~CVn (Kopacki and Pigulski 1995) and
V879~Aql (Kopacki and Pigulski 1998) appeared to be W~UMa-type
eclipsing binaries.  Continuing this program, one of us (A.B.)
observed V357~Her during six nights in 2001.  The new photometry shows
clearly that V357~Her is a W~UMa-type eclipsing binary as well.  This
was already suggested by Schmidt et al.~(1995), but because
their conclusions relied on only 15 $V$ and $R$-filter data points
covering mainly the vicinity of the maximum light, the star was still
present in the newest catalogue of $\delta$~Scuti stars
(Rodr\'{\i}guez et al.~2000).

It should be noted that a program of observing neglected HADS is now
conducted also by another group of observers (Van Cauteren and 
Lampens 2001 and references therein) and resulted in some
reclassifications too.

The new photometric observations of V357~Her have been carried out
during six nights between May 21 and August 2, 2001, in the
observatory of the Institute of Experimental Physics of the University
in Bia{\l }ystok.  The telescope was 30-cm Meade LX\,200
Schmidt-Cassegrain equipped with a Pictor 416\,XTE CCD camera of 768
$\times$ 512 pixels covering a 7$\farcm$7 $\times$ 5$\farcm$1
field-of-view.  No filter was used and the exposure time was 60
seconds for all frames.  A total of 377 frames have been taken.  They
were calibrated in a standard way and then the magnitudes of stars
were derived by means of the Daophot package of Stetson (1991).

In order to know the colours of nearby stars, the field of V357\,Her
was also observed on one night, August 11, 2001, in the Bia{\l }k\'ow
Observatory of the Wroc{\l }aw University.  The same equipment and 
filters as described by Kopacki
and Pigulski (1998) were used.  In total, five $V$-filter and five
$I_{\rm C}$-filter frames with 100-s exposure time were taken.  The
resulting colour-magnitude diagram helped us to choose the comparison
stars.  Moreover, since Bia{\l }k\'ow filters match the Johnson $V$
and Cousins $I_{\rm C}$ passbands quite well, using the colour
difference between a red and a blue star from the field, we found the
effective wavelength of the Bia{\l }ystok observations to be 670~nm.
This value was used later in the fits.

Prior to the analysis, some datapoints with largest errors have been
rejected and the remaining 322 differential magnitudes of V357~Her
with respect to two nearby comparison stars of similar colour index
have been formed.  These differential magnitudes are plotted in the
phase diagram in Fig.~1.  The magnitudes are available through the
IBVS web site as a 5280-t2.txt file.

\IBVSedata{5280-t2.txt}

\IBVSfig{8cm}{5280-f1.eps}{Differential Bia{\l }ystok CCD observations of
V357~Her (dots), phased with the period of 0.28204111~d.  The solid
line is the synthetic light curve corresponding to the best solution
given in Table 3.  Below, the residuals from the fit are plotted in
scale.}

\vspace*{-0.8cm}

\centerline{Table 1. Times of the primary minimum of V357~Her}
\vskip 3mm
\begin{center}
\begin{tabular}{crrl}
\hline\noalign{\smallskip}
Time of minimum & Epoch & $O-C$ &  Source of\\
HJD 2\,400\,000.+& \multicolumn{1}{c}{$E$} & \multicolumn{1}{c}{[d]} &
data \\ \noalign{\smallskip}\hline\noalign{\smallskip}
48\,535.6087 $\pm$ .0013 &     0 & 0.0000 & Schmidt et al.~(1995)\\
52\,051.5332 $\pm$ .0002 & 12466 & 0.0000 & this paper \\
52\,055.4823 $\pm$ .0004 & 12480 & +0.0005 & this paper \\
52\,061.4050 $\pm$ .0005 & 12501 & +0.0004 & this paper \\
52\,066.4812 $\pm$ .0004 & 12519 & $-$0.0002 & this paper \\
52\,076.3518 $\pm$ .0005 & 12554 & $-$0.0010 & this paper \\
52\,124.5818 $\pm$ .0003 & 12725 &  0.0000 & this paper \\
\noalign{\smallskip}\hline
\end{tabular}
\end{center}



The preliminary value of the orbital period has been obtained by means
of the Fourier periodogram.  Using Wilson-Devinney (WD) code (Wilson
and Devinney 1971; Wilson 1991) we next derived the preliminary
synthetic light curve, which was fitted to the observations of each of
the six nights separately.  The times of minimum derived from these fits
are given in Table 1.

In addition, we recalculated the time of minimum light for Schmidt
et al.~(1995) data, using the same procedure.  Since these
observations were made in two filters, $V$ and $R$, the
corresponding entry in Table 1 is the mean of the minimum in $V$ and $R$.

A linear regression of all times of minimum light given in Table 1
with weights inversely proportional to the r.m.s.~errors, yielded the
following ephemeris (the number in parentheses denotes the
r.m.s.~error of the last digits):
\begin{equation}
\hbox{T$_{\rm min}$ I = HJD\,2448535.6087(15) + 0.28204111(12)}
\times E,
\end{equation}
where $E$ is the number of cycles elapsed from the initial epoch. The $O-C$ values listed in Table~1 were calculated with this ephemeris.

\IBVSfig{8cm}{5280-f2.eps}{The smallest values of the sum of weighted
squares of the residuals (SWSR) plotted as a function of the assumed
mass ratio $q$.}

Having refined the value of the orbital period, we tried to derive the
parameters of the system.  In order to do this, we used the WD
differential correction program.  Each data point was assigned a
weight proportional to the reciprocal of square of the Daophot error.
Since for V357~Her the mass ratio, $q = M_2/M_1$, is not known, it was
derived first from the WD fits in a most common way, that is, from a
series of the best fits with assumed value of $q$.  Fig.~2 shows the
results of the fits, where we used the sum of weighted squares of the
residuals (SWSR) as the indicator of the quality of the fit.  In these
calculations we assumed the overcontact geometry and the mean surface
temperature of star 1 (star 1 is, by definition, eclipsed near phase
0.0), $T_1$ = 4800~K.  Moreover, bolometric albedos, $A_1$ = $A_2$ =
0.5 (Ruci\'nski 1973) and gravity darkening coefficients, $g_1$ =
$g_2$ = 0.32 (Lucy 1967) were adopted.  Limb darkening coefficients,
$x_1^{\rm bolo}$ = $x_2^{\rm bolo}$ = 0.54 and $x_1$ = $x_2$ = 0.66
for $\lambda_{\rm eff}$ = 670~nm were taken from the tables presented
by Van Hamme (1993).  The calculations were performed for $q$ in the
range between 0.25 and 5.0 (Fig.~2).  Only five parameters were
iterated:  phase of the primary conjunction, $\phi_0$, inclination,
$i$, average temperature of star 2, $T_2$, surface potential,
$\Omega_1$ = $\Omega_2$, and relative monochromatic luminosity of star
1, $L_1$.  Since for some $q$ there were problems with convergence,
200 iterations were performed for each $q$ and then the solution with
lowest SWSR was adopted as the best.  The lowest values of SWSR
achieved during 200 iterations are plotted in Fig.~2.  It is clearly
seen that the smallest SWSR, that is, the best solution, is obtained
for $q \approx$ 3.5.

\vskip 0.5cm

\centerline{Table 3. Parameters of the best-fit synthetic light curve of
V357~Her}
\vskip 3mm
\begin{center}
\begin{tabular}{cc}
\hline\noalign{\smallskip}
Parameter & Value\\
\noalign{\smallskip}\hline\noalign{\smallskip}
Inclination, $i$ & 83$\fdeg$1 $\pm$ 0$\fdeg$3 \\
Effective temperature of star 1, $T_1$ & 4800~K (assumed) \\
Effective temperature of star 2, $T_2$ & 4577 $\pm$ 5~K \\
Surface potential, $\Omega_1$ ( = $\Omega_2$) & 7.295 $\pm$ 0.005 \\
Mass ratio, $q$ & 3.572 $\pm$ 0.006 \\
Relative luminosity of star 1 at $\lambda_{\rm eff}$ = 670~nm, $L_1/(L_1+L_2)$
& 0.2880 $\pm$ 0.0012 \\
Overfill factor, $f = (\Omega_{\rm in}-\Omega_1)/(\Omega_{\rm
in}-\Omega_{\rm out})$ & 0.11 $\pm$ 0.02 \\
\noalign{\smallskip}\hline
\end{tabular}
\end{center}

In order to get the final solution we started iterations with the
solution for $q$ = 3.5 setting $q$ as an additional free parameter.
In addition, we increased the number of the grid elements on the
components' surfaces and used model atmospheres in computing monochromatic
fluxes.  The final solution is given in Table 3. Please note, that the WD~code
underestimates the errors (see e.g., Maceroni and Rucinski 1997), so the errors
in Table~3 should be at least doubled in order to be realistic. The synthetic
light curve is overplotted in Figure~1. One can see that the residuals are
not perfectly flat and some systematic differences do remain.  We have
checked the differential magnitudes between other bright stars in the field of
V357~Her and found that the changes at this level can be attributed 
to some instrumental
effects.  The standard deviation of the residuals equals 0.013~mag. 

In V357~Her the hotter component is smaller and it is occulted in the
primary (deeper) minimum.  The temperature difference is large ($\sim$ 220~K)
indicating a poor thermal contact. 

Using the positions of 11 stars from the Hubble {\it Guide Star Catalogue},
we derived the equatorial coordinates of V357~Her with an accuracy to within
0$\farcs$2: 
$\alpha_{\rm 2000.0}$ = 18$\hr$44$\mm$31$\fsec$86,
$\delta_{\rm 2000.0}$ = +12$\deg$55$\arcm$31$\farcs$1.


{\bf Acknowledgements.} A.B.~thanks Prof.~B.\,Paczy\'nski for
donating the CCD camera used to obtain the observations presented in
this paper.


\vspace*{-0.2cm}

\references

G\"otz, W., Wenzel, W., 1956, {\it Ver\"off.~Sternwarte Sonneberg},
        Band {\bf 2}, Heft 5.

Kopacki, G., Pigulski, A., 1995, {\it Acta Astron.}, {\bf 45}, 753.

Kopacki, G., Pigulski, A., 1998, {\it Acta Astron.}, {\bf 48}, 747.

Lucy, L.B., 1967, {\it Zeitschrift f\"ur Astroph.}, {\bf 65}, 89.

Maceroni, C., Rucinski, S.M., 1997, {\it PASP}, {\bf 109}, 782

Rodr\'{\i}guez, E., L\'opez-Gonz\'alez, M.J., L\'opez de Coca, P., 2000, {\it ASP Conf. Ser.}, {\bf 210}, 499.

Ruci\'nski, S.M., 1973, {\it Acta Astron.}, {\bf 23}, 79.

Schmidt, E.G., Chab, J.R., Reiswig, D.E., 1995, {\it AJ}, {\bf 109}, 1239.

Stetson, P.B., 1987, {\it PASP}, {\bf 99}, 191.

Van Cauteren, P., Lampens, P., 2001, {\it Comm. in Asteroseismology}, {\bf 140}, 66.

Van Hamme, W., 1993, {\it AJ}, {\bf 106}, 2096.

Wilson, R.E., 1990, {\it ApJ}, {\bf 356}, 613.

Wilson, R.E., Devinney, E.J., 1971, {\it ApJ}, {\bf 166}, 605.

\endreferences


\end{document}

