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\IBVShead{5759}{26 March 2007}

\IBVStitletl{Eleven More Eclipsing Systems with Apsidal Motion}{ in the Large Magellanic Cloud}

\IBVSauth{Michalska, G.}

\IBVSinsto{Instytut Astronomiczny Uniwersytetu Wroc{\l}awskiego, 
Kopernika 11, 51-622 Wroc{\l}aw, Poland\\ e-mail: michalska@astro.uni.wroc.pl}

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\begintext

With the bulk of time-series photometric data coming from the long-term, mainly microlensing
surveys (OGLE, MACHO, EROS, ASAS, NSVS and others), different properties of eclipsing binaries
can be studied statistically and confronted with the theory of binary star formation and
evolution. As these surveys cover both our Galaxy and Magellanic Clouds, the properties of
eclipsing binaries in the environment of different metallicity can be examined.  There are
already many examples of the use of the large photometric databases for binary star studies
(e.g., Paczy\'nski et al., 2006; Derekas et al., 2007b) but the information included in these
databases is still far from being exploited.

Apsidal motion, a phenomenon observed in eccentric systems, can be used to test internal
structure of components (Claret \& G\'{\i}menez, 1993; Claret, 1999) or even to derive their
parameters (e.g., Benvenuto et al., 2002). Typically, apsidal periods are at least decades
long and thus require very long observing runs. Photometric surveys we listed above, many of
them still ongoing, are therefore ideal for detection and monitoring of this phenomenon.

In our study of detached eclipsing binaries in the Large Magellanic Cloud (LMC) that are
suitable for distance determination (Michalska \& Pigulski, 2005, hereafter Paper I), 98
systems were presented, of which fourteen showed apsidal motion clearly. However, a more
detailed analysis led us to the detection of eleven more systems in the sample we studied. In
these new systems, the apsidal motion is not so well pronounced as in those found earlier
albeit still detectable.  Thus, in the present paper, we update the list of eclipsing binaries
with apsidal motion in the LMC{}.  A discovery of about 40 systems with apsidal motion in the
LMC was also recently announced by Derekas et al.~(2007a).  They used MACHO microlensing
survey as the source of data.

Like in Paper I, the main source of the data we used was the OGLE-II $I$-band photometry of
\.Zebru\'n et al.~(2001) supplemented by the two-colour photometry from the MACHO (Allsman \&
Axelrod, 2001)  and EROS (Grison et al., 1995) sources for stars in common. The light curves
in all bands were analyzed simultaneously by means of the improved version of the
Wilson--Devinney (WD) program (Wilson \& Devinney, 1971; Wilson, 2001).

\IBVSfig{9cm}{5759-f1.eps}{The $O-C$ diagrams for 11 systems with apsidal motion. The filled
and open circles denote the primary and secondary times of minimum, respectively}
\IBVSfigKey{5759-f1.eps}{eclipsing systems in the Large Magellanic Cloud}{O-C diagram}

\IBVSfig{9cm}{5759-f2.eps}{The eccentricities of EA-type binaries in the LMC plotted against:
the logarithm of orbital period (a), the sum of fractional radii (b), and longitude of
periastron, $\omega$ (c). Systems with apsidal motion we found are plotted as open circles (14
systems from Paper I)  and open squares (this paper). The remaining points are for systems in
which apsidal motion was not detected}
\IBVSfigKey{5759-f2.eps}{EA type binaries in the Large Magellanic Cloud}{eccentricity}

\vspace{-22pt}
The detection of the apsidal motion was made in the same way as in Paper I. First, the data
were divided into several subsets. For each subset the inclination, the phase shift, the
eccentricity, $e$, the longitude of periastron, $\omega$, the temperature of the secondary,
surface potentials and the luminosity of the primary component were adjusted with the WD
program. Then, the mean values of the $e$ and $\omega$ were calculated. Next, the WD program
was run separately for each subset with $e$ and $\omega$ fixed and the phases of primary and
secondary minimum were derived from the best fit. These phases were transformed into times of
minimum closest to the mean epoch of all observations in a given subset.  The individual times
of minimum were used in the same way as explained in Paper I to derive mean orbital period,
$P_{\rm mean}$, and initial epoch, $T_{\rm 0,mean}$, which are listed in Table~1 for all
eleven systems. In Fig.~1, the $O-C$ values calculated using $P_{\rm mean}$ and $T_{\rm
0,mean}$, are plotted. The numbers in the first column of Table~1 follow designation of stars
used in Paper I. The longitudes of periastron passage, $\omega$, are given for epoch
HJD\,2450500.0.


\begin{table}[t]
\centering
\caption{Parameters for eleven new systems with apsidal motion}
\begin{tabular}{rlcrccc}
\noalign{\smallskip}\hline\noalign{\smallskip}
 & OGLE & $e$ & \multicolumn{1}{c}{$\omega$} & $\dot\omega$ & \multicolumn{1}{c}{$P_{\rm mean}$}
& $T_{\rm 0,mean}$ \\
Star & designation & & \multicolumn{1}{c}{[\deg]} & [\deg/year] & [d] & [HJD\,244\dots] \\
\noalign{\smallskip}\hline\noalign{\smallskip}
\#8  & 05350218-6944178	& 0.081 & 323 & 4.16 $\pm$ 0.26 & 2.989470 & 9292.7814\\
\#19 & 05371417-7020015	& 0.083 & 150 & 4.40 $\pm$ 0.29 & 3.256681 & 9184.2182\\
\#20 & 05164453-6932333	& 0.202 & 280 & 0.62 $\pm$ 0.04 & 5.603488 & 9053.8272\\
\#39 & 05250946-7004226	& 0.069 &  63 & 3.0 $\pm$ 0.3 & 3.625506 & 9021.9995\\
\#40 & 05404159-6959014	& 0.094 & 229 & 8.6 $\pm$ 0.7 & 2.009973 & 9668.2172\\
\#67 & 05312473-6925281	& 0.124 & 440 & 4.34 $\pm$ 0.25 & 2.536666 & 9048.9340\\
\#78 & 05121869-6858325	& 0.048 & 215 & 4.9 $\pm$ 0.7 & 2.390521 & 9102.2382\\
\#84 & 05221500-6938483	& 0.322 & 257 & 1.17 $\pm$ 0.05 & 4.722937 & 9054.2332\\
\#85 & 05203518-6934378	& 0.119 & 151 & 3.5 $\pm$ 0.4 & 2.117476 & 9120.6445\\
\#90 & 05264527-6944045	& 0.399 & 262 & 0.20 $\pm$ 0.03 & 6.536149 & 9069.1179\\
\#96 & 05181271-6935245	& 0.107 & 157 & 3.8 $\pm$ 0.4 & 2.575571 & 9071.1041\\
\noalign{\smallskip}\hline
\end{tabular}
\end{table}

In Fig.~2 we also show how the parameters of systems with apsidal motion compare with those of
all sample of 98 stars studied in Paper I.  As expected, for a given eccentricity, they
usually have the shortest orbital period (Fig.~2a) or the largest sum of relative radii
(Fig.~2b). We have already explained in Paper I that the selection effects cause systems with
detected apsidal motion tend to group around $\omega \sim 90\deg$ and 270\deg.

{\bf Acknowledgement.} The work was supported by the MNiI grant 1 P03D 016 27.

\vspace{-12pt}

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