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\IBVShead{5239}{19 February 2002}

\IBVStitle{THE PRIMARY MINIMUM OF OW GEMINORUM IN 2002}


\IBVSauth{DEREKAS, A.$^{1,4}$; KISS, L. L.$^{1,4}$; SZAB\'O, GY. M.$^{1,4}$; F\H{U}R\'ESZ, G.$^{1,4}$; S\'ARNECZKY, K.$^{2,4}$; HEINER, ZS.$^{3,4}$}

\IBVSinst{Department of Experimental Physics and Astronomical Observatory,
University of Szeged,\\e-mail: {\tt l.kiss@physx.u-szeged.hu}}

\IBVSinst{Department of Physical Geography, ELTE University, Ludovika t\'er
2., Budapest, H-1088 Hungary}

\IBVSinst{Department of Optics \& Quantum Electronics, University of Szeged,
POB 406, Szeged, H-6701 Hungary}

\IBVSinst{Guest Observer, Piszk\'estet\H{o} Station, Konkoly Observatory,
Hungary}

\SIMBADobj{OW Gem}

\IBVStyp{ EA }

\IBVSabs{We report CCD photometry of OW Geminorum carried out between} 
\IBVSabs{Dec. 17, 2001 and Jan. 11, 2002.} 
\IBVSabs{The observations cover the primary mininum, which}
\IBVSabs{occurred in the predicted moment. The next secondary}
\IBVSabs{minimum is expected between Oct. 8 and Nov. 7, 2002}
\IBVSabs{whose multicolour photometry would enable more accurate photometric }
\IBVSabs{modelling of the system.}

\begintext

The light variation of the long-period eccentric Algol-type eclipsing 
binary OW~Geminorum (=HDE~258878=BD$+17\deg1281$, sp. type F2 Ib-II, 
$V_{\rm max}$=8.2 mag, $\Delta$$V$=1.8 mag, P$\approx$1260 days) 
was discovered in 1988 by D.H. Kaiser during his photographic nova search 
(Kaiser et al. 1988). The first period determination was 
given by Kaiser (1988), while the high eccentricity was indicated
for the first time by the secondary minimum observations of
Williams (1989). Since the discovery, two primary minima, the first in 1991 
(Williams \& Kaiser 1991, Pravec 1992, Hanzl et al. 1993) and 
the second in 1995 (Hager 1996) were covered by visual, photoelectric and 
CCD observations. The most thorough analysis of the system was
presented by Griffin \& Duquennoy (1993, hereafter referred to as GD93),
mostly based on radial 
velocity measurements. There is very few multicolour photometry in
the literature and this fact initiated our observational efforts 
addressing the primary minimum in early 2002.

The ephemeris given by Williams \& Kaiser (1991) predicted this
year's minimum to occur on January 3, 2002. Our CCD photometric 
observations were carried out on 13 nights between Dec. 17, 2001 
and Jan. 11, 2002. The data were obtained in two observatories.
$BV(RI)_{C}$ photometry was done at Szeged Observatory with the 
0.4-m Cassegrain telescope equipped with an SBIG ST--9E CCD camera
(512$\times$512 pixels). The exposure times were between 5 and 60 seconds,
depending on the actual filter and target brightness. The majority of the 
data was recorded with the 60/90/180~cm Schmidt telescope 
mounted at the Piszk\'estet\H{o} Station of the Konkoly
Observatory. The detector was a Photometrics AT200 CCD camera 
(1536$\times$1024 pixels, KAF-1600 chip). Here the exposure times 
ranged from 2 to 20 seconds.

All data were reduced with standard tasks in IRAF\footnote{IRAF is distributed by
the National Optical Astronomy Observatories, which are operated by the
Association of Universities for Research in Astronomy, Inc., under
cooperative agreement with the National Science Foundation.}. We made aperture 
photometry with IRAF/DAOPHOT. Two nearby stars, already suggested 
for comparisons by Terrell et al. (1994), were chosen as comparison
and check stars (comp = GSC 1332-0578, $V$=9.90 mag, $B-V$=+0.30 mag, 
check = SAO~95777, $V$=9.05, $B-V$=+0.230 mag, Hanzl et al. 1993). We 
used the fainter star as comparison because a 6-hour long continuous
$B$-band photometry on Dec. 17, 2001 suggested slight (and ambiguous) 
changes of the brighter star at a level of 0.02-0.03 mag. Although 
later data did not confirm this suspected variability, we have preferred 
the fainter star as the main comparison. To determine the standard
transformation coefficients for both instruments, we obtained $BV(RI)_{C}$ band
images of the open cluster M67, containing a widely used photometric standard 
sequence (Chevalier \& Ilovaisky 1991). The estimated 
photometric accuracy based on the scatter of the standardized 
``comp minus check'' data is $\pm$0.03 mag in $B$, $\pm$0.02 mag in $V$ and 
$\pm$0.01 mag in $R_{C}$ and $I_{C}$ bands.
The whole dataset consists of 15 points in $B$ and $V$, 11 points in $R_{C}$ and 
13 points in $I_{C}$ band. All photometric data are presented in Table\ 1
and the light curves are plotted in Fig.\ 1.


\begin{table}
\caption{Differential magnitudes of OW~Gem relative to GSC 1332-0578 
(MJD=Hel.JD$-$2450000). ``Sz'' and ``Pi'' denote Szeged Observatory
and Piszk\'estet\H{o} Station, respectively.}
\vskip 3mm
\begin{center}
\begin{tabular}{lrrrrr}
\hline
MJD  &  $\Delta{B}$ &  $\Delta{V}$ & $\Delta{R}_{C}$ & $\Delta{I}_{C}$ & Obs. \\
\hline
2261.462 & $-$1.25 & $-$1.71 & $-$1.90 & $-$2.20 & Sz \\
2266.349 & $-$1.31 & $-$1.65 & $-$1.88 & $-$2.13 & Pi\\
2267.453 & $-$1.32 & $-$1.66 & $-$1.88 & $-$2.13 & Pi\\
2271.528 & $-$1.23 & $-$1.57 &        &         & Pi \\
2274.321 & $-$0.78 & $-$1.14 &        &         & Pi \\
2275.498 & $-$0.32 & $-$0.80 & $-$1.11 & $-$1.41 & Pi \\
2276.235 & $-$0.07 & $-$0.59 & $-$0.90 & $-$1.22 & Pi \\
2277.273 & 0.41 & $-$0.30 & $-$0.61 & $-$0.97 & Pi \\
2278.189 & 0.44 & $-$0.28 & $-$0.64 & $-$1.08 & Sz \\
2278.387 & 0.45 & $-$0.29 & $-$0.66 & $-$1.07 & Sz \\
2279.240 & 0.05 & $-$0.59 & $-$0.92 & $-$1.26 & Sz \\
2279.320 & 0.03 & $-$0.59 & $-$0.94 & $-$1.29 & Sz \\
2281.303 & $-$0.69 & $-$1.19 & $-$1.46 & $-$1.78 & Sz\\
2285.478 & $-$1.31 & $-$1.64 &         & $-$2.14 & Pi \\
2286.461 & $-$1.32 & $-$1.66 &         & $-$2.12 & Pi \\
\hline                                                              
\end{tabular}
\end{center}
\end{table}

We have determined the epoch of minimum by fitting low-order polynomials 
to the lower part of the light curves. Unfortunately, the weather 
did not permit observations on every night, thus the gaps 
make the epoch determination uncertain. The derived epoch is
Hel.JD 2452277.73$\pm$0.20 giving $O-C$=$-$0\fday29 (with the ephemeris 
given below). More accurate moment requires more data obtained at different geographic
longitudes which bridge the daily gaps in our light curves. 

We have collected all available times of minimum from Williams \& Kaiser (1991),
Hanzl et al. (1993) and Hager (1996) in order to construct the updated 
$O-C$ diagram. It is completely linear and the points scatter around a straight
line which has a non-significant slope of 0.01$\pm$0.02. Therefore, we conclude
that the present ephemeris

\begin{center}
${\rm Hel.JD}_{\rm min}=2449760.857+1258.59 \times E$
\end{center}

\noindent can be used to predict further minima.

\IBVSfig{10cm}{5239-f1.eps}{Standardized differential light curves of OW~Gem
with the adopted model light curves.}

\vspace*{-0.4cm}
In order to gain some insight into the physical nature of the components,
we have tried to fit the observed light curves with a simple model. 
GD93 presented a detailed analysis of the 
system, including a light curve fit of the primary minimum observed 
by visual observers. We wanted to check whether their model can be 
extended to describe multicolour data, too. For this purpose, we computed
model light curves with a programme written by LLK. The programme
was not developed to match a light curve by automatic iteration, but 
calculates light variations for a given model and enables the user to
adjust the parameters of the model until a satisfactory convergence toward
the observed curves are not found. The adjustable parameters include 
the sizes (under spherical assumption) and temperatures (assuming
black-body radiation) of the two stars, and the minimum projected 
distance between the centers of the model stars (``impact parameter'').
The relative transverse velocity is fixed by the known orbital
elements. Standard linear limb-darkening coefficients for ${\rm log}~g=1.0$,
T$_{\rm eff}$=7250 K and 4500 K, computed for the $BV(RI)_C$ photometric bands,
were taken from Diaz-Cordoves et al. (1995) and Claret et al. (1995).
Standard $BV(RI)_C$ magnitudes were calculated by multiplying
the computed fluxes with the filter transmission functions (Bessell 1990)
and integrating over wavelength.

The spectral types of the components are known fairly accurately
thanks to this direct detection in the spectrum (GD93).
The brighter primary is of spectral type F2 Ia-II, while the secondary 
is a late G-type star (G8 according to GD93).
We have adopted the following geometric parameters determined by
GD93: R$_1$=30 R$_\odot$, R$_2$=35 R$_\odot$.
The secondary moves with 66 km~s$^{-1}$ relative to the primary and 
the impact parameter is 10 R$_\odot$. The initial temperatures 
were T$_1$=7300 K and T$_2$=4600 K, corresponding to F2 and G8 of 
luminosity class I-II (Carroll \& Ostlie 1996, Appendix E). 
Good fits (solid lines in Fig.\ 1) could be obtained for the same geometry
with slightly different
temperatures: T$_1^{\rm fit}$=7000$\pm$100 K, T$_2^{\rm fit}$=4900$\pm$100 K.
(We noticed some asymmetry in the ascending branches of the light curves, 
most visible in $R_C$ and $I_C$. Independent data are needed to decide whether 
it is real.)
The corresponding luminosities are L$_1\approx$1950 L$_\odot$, L$_2\approx640$
L$_\odot$, giving visual absolute magnitudes M$_{V}^1=-3.5$ mag and 
M$_{V}^2=-2.0$ mag. These values are in good agreement with those of 
GD93, the only difference is the slightly 
hotter secondary. We conclude that presently available data are compatible
with an F2-3 Ib-II primary and G4-5 IIb secondary.

The next secondary minimum will occur between Oct.~8 and Nov.~7,~2002. 
We urge CCD observers to follow this event by observing in as many photometric
bands as possible. It is expected that multicolour coverage of both minima 
would improve the photometric modelling of the components giving a more 
accurate physical description of this interesting system.

\vskip0.3cm
{\small \noindent This research was supported by the ``Bolyai J\'anos'' 
Research Scholarship of LLK from the Hungarian Academy of Sciences,
FKFP Grant 0010/2001, Hungarian OTKA Grants \#T032258 and
\#T034615 and Szeged Observatory Foundation. The warm 
hospitality of the staff of the Konkoly Observatory is 
gratefully acknowledged.
The NASA ADS Abstract Service was used to access data and references.
This research has made use of Simbad Database operated at CDS, Strasbourg,
France.}

\references

Bessell, M. S., 1990, {\it PASP}, {\bf 102}, 1181

Carroll, B. W., Ostlie, D. A., 1996, {\it An Introduction to Modern
Astrophysics}, Addison-Wesley Pub., New York

Chevalier, C., Ilovaisky, S. A., 1991, {\it A\&AS}, {\bf 90}, 225

Claret, A., Diaz-Cordoves, J., Gimenez, A., 1995, {\it A\&AS}, {\bf 114}, 247

Diaz-Cordoves, J., Claret, A., Gimenez, A., 1995, {\it A\&AS}, {\bf 110}, 329

Griffin, R. F., Duquennoy, A., 1993, {\it The Observatory}, {\bf 113}, 53 (GD93)

Hager, T., 1996, {\it JAAVSO}, {\bf 24}, 9

Hanzl, D., Papousek, J., Chochol, D., Pravec, P., 1993, {\it IBVS}, No. 3821

Kaiser, D. H., 1988, {\it IBVS}, No. 3233

Kaiser, D. H., Baldwin, M. E., Williams, D. B., 1988, {\it IBVS}, No. 3196

Pravec, P., 1992, {\it IBVS}, No. 3792

Terrell, D., Kaiser, D. H., Williams, D. B., 1994, {\it IBVS}, No. 4102

Williams, D. B., 1989, {\it JAAVSO}, {\it 18}, 7

Williams, D. B., Kaiser, D. H., 1991, {\it JAAVSO}, {\bf 20}, 231

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