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\IBVShead{5868}{17 December 2008}

\IBVStitletl{Two pairs of interacting EBs}{ towards the LMC in the OGLE database}

\IBVSauth{Ofir, A.$^1$}

\IBVSinst{School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel}

\SIMBADobj{OGLE J051343.14-691837.1}

\IBVSabs{A single point source at the OGLE LMC database shows the characteristics}
\IBVSabs{of two superimposed eclipsing binaries (EBs). The two EBs happen to have}
\IBVSabs{periods very close to the 3:2 resonance. The telescope's small PSF and}
\IBVSabs{the apparent resonance between the two EBs raises the suspicion that}
\IBVSabs{this is not chance alignment but rather a compact hierarchical system}
\IBVSabs{of two pairs of interacting EBs in 3:2 resonance.}

\begintext

Manual browsing through the online OGLE LMC database\footnote{http://ogle.astrouw.edu.pl/} (Wyrzykowski \textit{et al.} 2003) 
revealed that eclipsing binary OGLE 051343.14-691837.1 ($P_1=3.57798$\,d) is significantly more noisy than other stars with similar 
brightness, and indeed another EB was subsequently found in it's residuals ($P_2=5.36655$\,d). This second EB has a period of 
almost exactly 1.5 times the period of the first EB. To better disentangle the two signals we simultaneously fitted a truncated 
Fourier series with $N$ terms ($N=22$) for each period, plus a zero-point term, for a total of 45 fitted coefficients. 
We then rejected outliers and repeated untill convergence. This allowed us to better visualize both EBs (see Fig.~1) 
and it is easy to see that the two signals are not different harmonics of the same system but rather two distinct EBs. Since 
OGLE's telescope PSF is rather small, and because of the apparent resonance between the two binaries, we believe it is highly 
unlikely that this is chance alignment, and that the more probable explanation is of a rather compact hierarchical system of 
two pairs of EBs in 3:2 resonance. Interestingly, it seems that all 4
stars are rather massive as both EBs show very significant ellipsoidal
variation. With this scenario in mind, the fact that both pairs of stars are EBs means that some degree of 
co-planarity also exist - further supporting the interacting-pairs hypothesis.


\reference

Wyrzykowski, L. et al. 2003, Acta Astron., 53, 1

\endreferences

\pagebreak

\IBVSfigNoCaption{7.5cm}{5868-f1a.eps}
\IBVSfig{7.5cm}{5868-f1b.eps}{The red solid lines are smoothed light curves using a  
truncated Fourier series - see text for details (for each EB the other  
EB signal is removed). Outliers are marked in red boxes.}


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