\documentstyle[twoside, epsf]{article}
\input ibvs2.sty

\begin{document}

\IBVShead{5445}{23 July 2003}

\IBVStitletl{Multicolor Observations of the Primary and Secondary}{ Eclipses of OW Geminorum }

\IBVSauth{Miko{\l}ajewski, Maciej; Ga{\l}an, Cezary; Graczyk, Dariusz}
%\IBVSauth{Miko{\L}ajewski, Maciej; Ga{\L}an, Cezary; Graczyk, Dariusz}

\IBVSinsto{Uniwersytet Miko{\l}aja Kopernika, Centrum Astronomii, ul. Gagarina 11, Pl-87-100 Toru\'n, Poland; \\ e-mail: mamiko@astri.uni.torun.pl (MM); cgalan@astri.uni.torun.pl (CG); weganin@astri.uni.torun.pl (DG)}


\SIMBADobjAlias{OW Gem}{HD 258878}
\IBVStyp{ E }
\IBVSkey{photometry}
\IBVSabs{Multicolor observations of long period binary OW Gem are reported:}
\IBVSabs{five bands photometry during the primary eclipse in 1995 and seven}
\IBVSabs{bands photometry during the secondary eclipse in 2002.}
\IBVSabs{Simple model of both eclipses gives most probably stellar}
\IBVSabs{temperatures about 7100K and 4950K and the orbital inclination close}
\IBVSabs{to 89 degrees.}


\begintext

OW Gem is a unique, long-period, well-detached, eclipsing binary system,
composed of two evolved supergiant stars. Their spectroscopic orbits are well
known (Griffin~$\&$~Duquennoy, 1993). The orbital period derived from
observations of 11 eclipses between 1902 and 1991 is 1258.59~days, {\it i.e.}
about 3.45~years (Williams~$\&$~Kaiser, 1991). Griffin (1993) described
8 similar long-period systems, containing two comparable giants with known
spectroscopic orbits, but only OW Gem shows eclipses, thus it remains the only
system with precisely known masses for both components.

Terrell et al. (1994) mounted an international photoelectric campaign to
observe this star during primary and secondary eclipses in 1995. We answered
their appeal but unfortunately, the analysis of this campaign exclude our
photometric data (Kaiser~et~al., 2002). In 1995 we observed OW Gem in
{\it UBVri} bands with the 60 cm Cassegrain reflector at Piwnice Observatory
near Toru\'n (Poland). We used a single-channel diaphragm photometer with an
unrefrigerated EMI 9558B photomultiplier. Our {\it UBV} response curves were
very close to the standard Johnson's system, whereas our broad {\it ri} bands had
significantly shorter mean wavelengths: $6390${\AA} and $7420${\AA},
respectively. HDE~258848 was chosen as a comparison star and GSC~1332:0578 as a
check star, both suggested by Terrell~et~al. (1994). The accuracy of our
measurements was about $\pm0\fmm03$ in {\it U}, $\pm0\fmm02$ in {\it BVr} and
$\pm0\fmm01$ in {\it i} bands. Our observations during the primary eclipse in 1995
are presented in Fig.~1. Unfortunately, we only obtained a few observational
points around the secondary eclipse in late 1995.

One year ago, Derekas et al. (2002) reminded us again of this star by
their observations of the primary eclipse at the turn of 2001$/$2002. So
we decided to observe the secondary eclipse, which took place only a few months
later, in October~2002, thanks to the large orbital eccentricity~($e=0.52$).
Before the publication of Kaiser et al. (2002), only three {\it V}
measurements, $0\fmm1$ below the average brightness of the star outside
the eclipse (Williams, 1989), were known as photometric signs of the
secondary eclipse.

In 2002, we made 24 measurements during 22 nights between Sept.~2 and Dec.~9
which gave quite a good time coverage of the secondary eclipse. The data were
obtained with the same telescope equipped with a cooled Burle/RCA C31034
photomultiplier. The standard Johnson-Cousins {\it UBV(RI)$_C$} system and two
intermediate-band interference filters (FWHM~$\approx100${\AA}), ``{\it h}"(located
at $H_{\beta}$) and ``{\it c}"(located in the continuum around $4804${\AA}) were
used.

\IBVSfig{6.0cm}{5445-f1.eps}{The {\it UBVri} light curves of OW Gem during
the primary eclipse in 1995 with the best fit model ($T_{hot}=7100{\rm K},
T_{cool}=4950{\rm K}, i=89\deg$). The neighbouring curves are shown with
different (filled and open) circles for clarity.}
%%\IBVSfigKey{5445-f1.eps}{OW Gem}{light curve}

\IBVSfig{11.5cm}{5445-f2.eps}{The {\it UBV(RI)$_C$} light curves of
OW Gem during the secondary eclipse in 2002 with the best fit model
($T_{hot}=7100{\rm K}, T_{cool}=4950{\rm K}, i=89\deg$). The
neighbouring curves are shown with different (filled and open) circles.
Crosses in the {\it V} light curve correspond to Williams' (1989)
observations shifted to epoch 2002 with period $1258\fday59$.}
%%\IBVSfigKey{5445-f2.eps}{OW Gem}{light curve}

The accuracy of our measurements was $\pm0\fmm03$ in ``{\it h}" and ``{\it c}",
$\pm0\fmm02$ in {\it UBVR$_C$} and $\pm0\fmm01$ in {\it I$_C$} bands. The same
comparison and check stars were utilized as in 1995. Fig.~2. presents our light
curves of the secondary eclipse. The time of the mid-eclipse was in an agreement
with the ephemeris given by Williams~(1989). The depth of the eclipse increases with
increasing wavelength from about $0\fmm08$ in the {\it B} up to $0\fmm2$ in the
{\it I$_C$}. Our data obtained during the primary eclipse in 1995 (Fig.~1) and
especially during the secondary eclipse in 2002 (Fig.~2) are good complements
of the observations previously collected by Kaiser et al. (2002) and  Derekas 
et al. (2002).

We have tried to fit a very simple model to our light curves. The sizes of both
stars were fixed at values given by Griffin $\&$ Duquennoy (1993):
$R_{hot}=30R_{\odot}$ and $R_{cool}=35R_{\odot}$. The limb darkening was
neglected and stellar fluxes were approximated as blackbodies. The adjustable
parameters were the effective temperatures of the hotter ($T_{hot}$) and the 
cooler ($T_{cool}$) components and the impact parameter $D$ which measures the
projected distance between the centres of stellar discs in the mid-eclipse
point. We took the timings from Kaiser et al. (2002): JD~2449760.6 for
the primary in 1995 and JD~2452570.9 for the secondary in 2002.

\IBVSfig{8cm}{5445-f3.eps}{The best fits of orbital inclination $i=i_{oc}$
found for the primary (continuous lines) and $i=i_{tr}$ for the secondary
(dashed lines) eclipses. Particular lines are signed by the temperature of
the hotter component $T_{hot}$. The most probable solutions (thick line)
are denoted by intersections between lines corresponding to opposite eclipses
(when temperatures and inclination derived from both minima are the same).}
%%\IBVSfigKey{5445-f3.eps}{OW Gem}{}

We have adopted three reasonable temperatures $T_{hot}$ for the F2~Ib-II
component: 6800K, 7100K, 7400K. The temperature of the G8~IIb
component $T_{cool}$ has been changed from 4500~K to 5500~K with a step of
100K.  For each pair of such temperatures we have found an impact parameter $D$
giving the best fit to our observation in all {\it UBVri} or {\it UBV(RI)$_C$}
bands simultaneously. As a best fit criterion we used the minimum of the sum of
normalized standard deviations in five bands
$\sigma^{e}_{U}/\sigma^{o}_{U}+\sigma^{e}_{B}/\sigma^{o}_{B}+
\sigma^{e}_{V}/\sigma^{o}_{V}+\sigma^{e}_{r/R_{C}}/\sigma^{o}_{r/R_{C}}+
\sigma^{e}_{i/I_{C}}/\sigma^{o}_{i/I_{C}}$, where indices `$e$' and
`$o$' correspond to fits during and outside the eclipse,
respectively. The narrow filters $h$ and $c$ were omitted in these calculations
in order to keep the same weights for both minima.

The spectroscopic orbit of Griffin $\&$ Duquennoy (1993) gives the distance
between the stars during transit to be about twice that during the occultation:
$r_{oc}=0.553a$ and $r_{tr}=1.096a$, where $a$ is semi-major axis. The orbital
inclination {\it i} can be derived separately for each eclipse:
$$ \tan i_{oc}=\frac{r_{oc} \sin i}{D_{oc}}=\frac{0.553 a \sin i}{D_{oc}}$$
$$ \tan i_{tr}=\frac{r_{tr} \sin i}{D_{tr}}=\frac{1.096 a \sin i}{D_{tr}}$$
where $D_{oc}$ and $D_{tr}$ are impact parameters during the occultation and
transit, respectively. Adopting a projected semi-major axis $a \sin
i=1052R_{\odot}$ (Griffin $\&$ Duquennoy, 1993) we have found the
inclination for different stellar temperatures fitted to both eclipses
separately (Fig.~3). Of course, the inclination obtained from primary and
secondary eclipses must be the same, {\it i.e.} $i=i_{oc}=i_{tr}$. This
requirement corresponds to that shown in Fig.~3 at the intersection between
the solutions $i=i_{oc}(T_{hot},T_{cool})$ and $i=i_{tr}(T_{hot},T_{cool})$
for occultation (primary) and transition (secondary), respectively. As a result,
the orbital inclination $i=89^{\circ}$ is almost constant and independent of
the stellar temperatures . This is a consequence of arbitrary assumed stellar
dimensions from Griffin $\&$ Duquennoy (1993) who found the same inclination.
They also estimated temperatures of both components of 7100K and 4800K.
Nevertheless, our calculations show the temperature ratio
$T_{hot}/T_{cool}\approx1.43\pm0.01$, which is significantly smaller than
$7100/4800~\approx~1.48$. Based on our values the secondary component is
likely to be hotter (about $4900-5000$K) which agrees with the suggestion by
Derekas et al. (2002). Synthetic light curves for our most probable parameters
are presented together with observations in Figs.~1~and~2.

We will search for better radii of both stars with the Wilson-Devinney
code, including limb darkenings and models of stellar atmospheres for the
calculations. Unfortunately, we need more and better
observations outside the eclipses, in particular, close to the periastron where
small deformations of stars cannot be excluded. Additionally, hot and cool spots
on the supergiants' convective surfaces are very likely, which can significantly
affects the solutions.

{\it Acknowledgements:} We are very grateful to Dr. Boud Roukema and to the referee,
Dr. Laszlo Kiss for their English improvements in the text. This study was supported by
KBN grant No~5~P03D~003~20.

\references

Derekas, A., Kiss, L. L., Szab\'o, Gy. M., F\H{u}r\'esz, G., S\'arneczky, K., Heiner,
Zs., 2002, {\it IBVS}, No. 5239

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

Griffin, R. F., 1993, {\it The Observatory}, {\bf 113}, 294

Kaiser, D. H., Henden, A. A, Dvorak, S., Garcia-Melendo, E., Gomez-Forrellad,
J. M., Howell, J. A., Koff, R. A., Kuebler, P., Lubcke, G. C., Martin, B.,
Nelson, R. H., Pullen, A. C., Terrell, D., West, D., Williams, D. B., Wolf,
G. W., Zissell, R. E., 2002, {\it IBVS}, No. 5347

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

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

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

\endreferences

\end{document}

