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\IBVShead{5719}{31 July 2006}

\IBVStitletl{GSC 2038.0293 is a new short-period}{ eclipsing RS CV\lowercase{n} variable}

\IBVSsubtitle{\rm (BAV MITTEILUNGEN NO. 177)}

\IBVSauth{Bernhard, K.$^{1,3}$; Frank, P.$^{2,3}$}

\IBVSinst{A--4030 Linz, Austria; e-mail: {\tt klaus.bernhard@liwest.at}}
\IBVSinst{D--84149 Velden, Germany; e-mail: {\tt frank.velden@t-online.de}}
\IBVSinst{Bundesdeutsche Arbeitsge\-mein\-schaft f\"{u}r Ver\"{a}nderliche 
Sterne e.V. (BAV), Munsterdamm 90,\\ \indent D--12169~Berlin, Germany}

\IBVStyp{ RSCVn }
\IBVSkey{RSCVn - CCD photometry, eclipsing binary - CCD photometry}

\IBVSabs{GSC 2038.0293 is a new eclipsing RS CVn variable, which has been}
\IBVSabs{discovered in the ROTSE1 database. A very short period of 0.495410}
\IBVSabs{days has been determined from the available ASAS3 and ROTSE1 data}
\IBVSabs{and our BVR and unfiltered CCD photometry in 2005 and 2006. Large}
\IBVSabs{changes in the light curve occur on timescales of a few weeks.}


\SIMBADobjAlias{1RXS J160248.3+252031}{GSC 2038-0293}


\begintext

During a programme of optical identification of X-ray sources from the
ROSAT all-sky survey bright source catalogue (Voges et al., 1999) with the
ROTSE1 database (Wo\'{z}niak et al., 2004) it was found that the
uncatalogued variable NSVS object ID 7869362 (=~GSC 2038.0293) was
coincident with the X-ray source 1RXS J160248.3+252031. Further details of the programme are presented in Bernhard et al.\ (2005).  GSC 2038.0293 has $V=10.62$ and $B-V=0.83$ from the Tycho-2 catalogue (H\o g et al., 2000), the 2MASS
catalogue gives $J-K = 0.612$ (Cutri et al., 2003).

Our observations were made using both a 20-cm Schmidt--Cassegrain telescope
and a Starlight XPress SX CCD camera with $BV$ filters (2005) and $BVR$
filters (2006) in Linz, Austria and a Flatfield Camera 576/2.0 with a
CCD camera OES-LcCCD12 and $IR$-cutting filter in Velden, Germany (2005 and
2006).  The comparison stars used were GSC 2038.0565 and GSC 2038.0663,
which were found to be constant within $< 0.03$ mag.

The following primary minima were observed in 2005 and 2006 (Table 1):

\begin{table}[h] 
\centerline{\normalsize Table 1: Times of primary minima of GSC 2038.0293 (HJD 245\dots)} 
\vskip 3mm
\begin{center}
\begin{tabular}{lcll}
\hline
minimum time & filter & observer & $O-C$ (d) \\
\hline
3566.433 (2) & $IR$ cutt. & Frank & $-0.002$ \\
3569.405 (2) & $V$ & Bernhard & $-0.003$ \\
3846.348 (2) & $IR$ cutt. & Frank & +0.005 \\
3877.555 (2) & $V$ & Bernhard & +0.002 \\
\hline
\end{tabular}
\end{center}
\end{table}

Figures in brackets denote rms errors in units of the last decimal, $O-C$
values were calculated with the ephemeris given below.

A Fourier analysis of the available data including ASAS3 (Pojmanski, 2002)
and ROTSE1 was performed to search for periodicity of the light
variations. The following ephemeris can be derived from the analysis with
the algorithm Period04 (Lenz and Breger, 2005):
$$
\begin{array}{r@{}r@{}l}
{\rm HJD_{MinI}} = 2453560.491 & {} + 0\fday495410 & {} \times E.\\
\pm 3 & \pm 1 \\
\end{array}
$$


The folded (and in $y$-direction shifted) light curves of the $BVR$ filtered
observations in May and June 2006 are given in Figure 1 and show an
amplitude of nearly 0.20 mag for the $B$ observations and of nearly 0.18 mag
for the $V$ and $R$ observations.

\IBVSfig{10cm}{5719-f1.ps}{Folded $BVR$ light curves of GSC 2038.0293 in the
observing season 2006}
\IBVSfigKey{5719-f1.ps}{GSC 2038-0293}{light curve}

The shape of the folded light curve with two minima of considerably
different width clearly identifies GSC 2038.0293 with a very short period
and heavily spotted RS CVn type star. This finding is also supported by
the X-ray identification, and the values of the Tycho-2 and 2MASS colours,
which point to a spectral type of late G or early K{}. $B-V$ and $V-R$
values of our observations in 2006 indicate a slight reddening of the
star, when it enters the minimum of the spotted light curve. The peak to
peak amplitude 
(i.e.\ the magnitude difference between the secondary and primary minima),
determined by
low order polynomial fitting, is for the $B$ band about 0.09 mag, for the $V$
and $R$ band only 0.07 and 0.06 mag. This is in good agreement with data
from literature, where a $\Delta R/\Delta V$ value of 0.90 for active
stars has been determined (Drake, 2006).

The period of 0.495410 days is very short for an RS CVn star. Only one of
206 binary systems of the second edition of the catalogue of
chromospherically active binary stars has a shorter period (XY UMa,
0.4789944 days; Strassmeier et al., 1993).

The folded light curves of ROTSE1, ASAS3 and our $V$-band data, which are
shifted for the different years, are given in Figure 2.

\IBVSfig{10cm}{5719-f2.ps}{Folded ASAS3 and ROTSE light curves (filled
circles) and our $V$ data (open circles) in 1999--2006}
\IBVSfigKey{5719-f2.ps}{GSC 2038-0293}{light curve}

\IBVSfig{8cm}{5719-f3.ps}{Peak to peak amplitude of the minimum of the
spotted light curve in the ASAS3 and ROTSE data (filled squares) and in
our $V$-band data (open squares) in 1999--2006}
\IBVSfigKey{5719-f3.ps}{GSC 2038-0293}{}

\vspace{-24pt}
ROTSE1 data are available for 1999 and 2000, ASAS3 $V$ data for 2003, 2004,
2005 and 2006 (filled circles). Our $V$-band data for 2005 and 2006 are
shown as open circles. The amplitudes of the $V$ and $R$ band are very similar
(see Figure 1). Therefore it can be assumed, that also ASAS3 ($V$) and
ROTSE1 amplitudes (near $R$ values) are roughly comparable.

It can be clearly seen, that the primary minimum has fairly the same
amplitude in 1999--2006, but the depth of the minimum of the spotted light
curve is changing to a large extent. The long-term changes of the light
curve are illustrated in more detail in Figure~3.

The amplitude of the spotted light curve shows
a clear variation in 1999--2006 with two clear maxima in the years 1999
and 2005. In 2005, the year of the highest variation, the minimum of the
spotted light curve was fainter than the primary (eclipsing)
minimum. We noticed considerable changes in the shape of the lightcurve on
timescales of a few weeks in our $B$, $V$ and unfiltered observations. This
resulted in an increased scatter near the minimum of the spotted light
curve in the ASAS and our data of that year (see Figure~2).

Though it is clear that more observations will be necessary to describe
the long-term activity of GSC 2083.0293, (cyclic?) variations on
timescales of 6--8 years seem to occur.  Similar cycles have also been
observed for other RS CVn stars (e.g.\ Berdyugina and Tuominen,
1998). We conclude that GSC 2038.0293 is a new RS CVn variable with one of
the shortest known periods and a dramatically changing light curve. We
hope that the present study will stimulate more observations of this
interesting, high activity star.

{\bf Acknowledgements:} The authors want to thank Dr.\ Christopher Lloyd and Dr.\
Konrad Dennerl for collaboration and suggestions. This research has made
use of the SIMBAD and VizieR databases operated at the Centre de Donn\'ees
Astronomiques (Strasbourg) in France.

\references

Berdyugina, S.V., Tuominen, I., 1998, {\it Astron.\ Astrophys.}, {\bf 336},
L25-L28

Bernhard, K., Lloyd, C., Berthold, T., Kriebel, W., Renz, W., 2005, {\it
IBVS}, No.\ 5620

Cutri, R.M., et al., 2003, 2MASS All-Sky Catalog of Point Sources,
University of Massachusetts and IPAC/California Institute of Technology

Drake, A.J., 2006, {\it AJ}, {\bf 131}, 1044

H\o g, E., Fabricius, C., Makarov, V.V., Urban, S., Corbin, T., Wycoff,
G., Bastian, U., Schwekendiek, P., Wicenec, A., 2000, {\it A\&A}, {\bf
355}, L27

Lenz, P., Breger, M., 2005, {\it Comm.\ in Asteroseismology}, {\bf 146}, 53

Pojmanski, G., 2002, {\it Acta Astronomica}, {\bf 52}, 397

Strassmeier, K.G., Hall, D.S., Fekel, F.C., Scheck, M., 1993, {\it Astron.\ Astrophys.\ Suppl.}, 
{\bf 100}, 173-225

Voges, W., et al., 1999, {\it Astron.\ Astrophys.}, {\bf 349}, 389, The
ROSAT all-sky survey bright source catalogue

Wo\'{z}niak, P.R., et al., 2004, {\it Astron.\ J.}, {\bf 127}, 2436,
Northern Sky Variability Survey: Public Data Release

\endreferences

\end{document}

