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\IBVShead{5991}{30 May 2011}

\IBVStitletl{V407 P\lowercase{eg} and LU V\lowercase{ir}:}{ Two Contact Binaries with Displaced Secondary Minima}


\IBVSauth{ZASCHE, P.}

\IBVSinsto{Astronomical Institute, Faculty of Mathematics and Physics, Charles University Prague, CZ-180 00 \\ Praha 8, V Hole\v{s}ovi\v{c}k\'ach 2, Czech Republic, e-mail: zasche@sirrah.troja.mff.cuni.cz}


\SIMBADobj{V407 Peg}
\SIMBADobjAlias{LU Vir}{HD 116914}
\IBVStyp{ CWA }
\IBVSkey{photometry}
\IBVSabs{Contact eclipsing binaries are always on circular orbit.} 
\IBVSabs{We present two rather unusual systems, where the secondary minimum is}
\IBVSabs{displaced from a phase 0.5 from the primary one.}
\IBVSabs{This is probably caused by strong asymmetry of the light curve.}


\begintext


The role of eccentric eclipsing binaries in modern stellar astrophysics is undisputed. 
There are a few binaries, however, which are not eccentric, but show displaced secondary minima nevertheless.
If one has no information about their light curve, one can consider the eccentricity only on
the basis of the $O-C$ diagram of all available minima of a particular system. Due to the
increasing number of CCD observations of minima, some systems were discovered with short period but displaced secondary minima.

The process of circularization is rather rapid in those binaries, which are close. Eccentric orbits
are common in well-detached binaries with longer periods, or in systems which are very young.
When the system is semi-detached or even contact, the nonzero eccentricity is extremely 
improbable. Therefore any discovery of a short period binary with displaced secondary minima (i.e. $\phi_2 \neq
0.5$ with respect to the primary) has to be considered very carefully.


The system {\bf V407 Peg} (= GSC 01720-00658, V = 9.28~mag, sp F1) is a typical W~UMa-type
eclipsing binary. It was discovered by Maciejewski et al. (2002), showing two minima of almost
equal depths. However even in the discovery paper it was noted that the secondary minimum is
located in phase 0.52. The secondary minima occurred about 0.015~days later than phase 0.5. This is
rather unusual for a W~UMa type binary.

A detailed analysis was done by Maciejewski et al. (2003) and Rucinski et al. (2008). These complete
analyses show that the inclination is about 73~deg and the masses are 1.53 and 0.39 for the primary
and secondary, respectively. The observed asymmetry of the LC was explained by a
photospheric hot spot. This spot also distorts both minima and also the maxima of the
LC (O'Connell effect).

We collected all available (published) times of minima of V407 Peg. For a prospective analysis we
need as many minima times as possible. Therefore, we also used the data for this target obtained by
the automatic photometric monitoring system ASAS (Pojmanski,~2002). The star was also observed by
the Pi of the Sky (Burd et al., 2005). Thanks to these observations, many new minima times were
derived. A collection of all available times of minima is given in Table 1. These minima follow a
linear ephemeris with the orbital period of 0.636883915~day.

Thanks to these minima one is able to study a potential long-term variation of the LC. The changing
position of the spot on the surface is able to modulate the shape of the minima, and therefore also
the position of primaries versus secondaries in the $O-C$~diagram (plotted in Fig. 1). As one can
see, the difference between primary and secondary minima is still visible and is not changing
significantly. Some of the points have rather large errors, but still one can speculate about some
year-to-year differences. For example the first data point from 2003 and the following ones from 2004
show a non-negligible difference. For a detailed evolution of the spot or other processes in the
system a more detailed year-by year LC analysis would be desirable.


\IBVSfig{5cm}{5991-f1.eps}{$O-C$\,diagram of V407\,Peg dots stand for primary and open\,circles for the secondary\,minima.}
\IBVSfigKey{5991-f1.eps}{V407 Peg}{O-C diagram}

The shape of the LC taken from ASAS data is plotted in Fig. 2. As one can see, both minima are
distorted and asymmetric. The bottom parts of minima (probably total eclipses) are not flat either, 
but rather inclined. As a result it is complicated to derive a precise time of minimum from
the observed data. Most of the often used methods for minima computation are based on symmetric
minima (e.g. Kwee-van Woerden, bisector chord method or polynomial fitting). Thus, using the data
points with different slope of ascending and descending branch makes the application of these methods rather
questionable and affects the result.

\begin{table}[h]
\begin{center}
\caption{Collected times of minima. The full table is available electronically. Sources: Br\'at et al. (2008, 2011); Nagai, K. (2003, 2004, 2005, 2008, 2009, 2011); Yilmaz et al. (2009), Perryman et al. (1997).}
\medskip
\begin{tabular}{cccccc}
\hline
Star   &HJD-2400000& Error & Type & Filter & Source  \\
\hline
 V407 Peg & 52534.2999  & 0.0014  & II & V & IBVS 5343   \\
 V407 Peg & 52534.6031  & 0.0008  & I  & V & IBVS 5343   \\
 V407 Peg & 52552.7715  & 0.0013  & II & V & IBVS 5343   \\
 ... & & & & &  \\
 LU Vir   & 48085.9734 & 0.0023 & I  & Hp & Hipparcos  \\ 
 LU Vir   & 48085.7456 & 0.0021 & II & Hp & Hipparcos  \\ 
 LU Vir   & 48727.8604 & 0.0025 & I  & Hp & Hipparcos \\
 ... & & & & &  \\
 \hline
\end{tabular}
\end{center}
\end{table}

\IBVSedata{5991-t1.txt}
\IBVSetable{5991-t1.tex}
\IBVSdataKey{5991-t1.txt}{V407 Peg ; LU Vir}{minima/maxima times}

\IBVSfig{5cm}{5991-f2.eps}{$V$ light curve of V407 Peg from ASAS.}
\IBVSfigKey{5991-f2.eps}{V407 Peg}{light curve}


 The second system is {\bf LU Vir} (= HD 116914 = HIP 65590, V = 7.78~mag, sp A0). It is also a W~UMa or
$\beta$~Lyrae star, but lacking a detailed analysis. Unlike the case of V407 Peg neither the LC nor the
radial velocity curve were studied and a period analysis is missing too. The orbital period of the
system is 0.492240615~day.


We collected all available published times of minima, these are given 
in Table 1. Some of the
published minima are not included because we used the original data and recalculated the minima
(these apply for the Hipparcos and ``Pi of the Sky" data). The $O-C$ diagram of these minima
is plotted in Fig. 3, where one can clearly see the difference between primary and secondary minima
over a time span of almost two decades. The precision of individual observations is questionable,
therefore a possible variation of displacement of secondary minima with time is still problematic
to detect. New and more precise observations are needed to confirm some possible variation of the
LC.

\IBVSfig{5cm}{5991-f3.eps}{$O-C$ diagram of LU Vir.}
\IBVSfigKey{5991-f3.eps}{LU Vir}{O-C diagram}

\IBVSfig{5cm}{5991-f4.eps}{$V$ light curve of LU Vir from ASAS.}
\IBVSfigKey{5991-f4.eps}{LU Vir}{light curve}


The light curve of LU Vir is asymmetric in the same way as that of V407 Peg. We can see the curve in Fig. 4,
where the primary minimum with its total eclipse is clearly distorted and the flat part is
inclined. Thus the derivation of minima times is affected by asymmetry and the use of standard
routines for minimum computation is problematic.

To conclude, in both systems the displaced secondary minima are very probably caused by asymmetry
of the light curve, which is not changing significantly over a time span of more than a decade. It
could be produced by an accreting hot spot from a stream of material flowing from one component to
another, which is a rather common phenomenon in such semidetached or contact binaries.

\vfill\eject

The use of SIMBAD, operated at CDS, Strasbourg, France, and of NASA's Astrophysics Data System
Bibliographic Services, are greatly acknowledged. We thank the ``ASAS" and ``Pi of the Sky" teams for
making all the observations publicly available. This work was supported by the Czech
Science Foundation grant no. P209/10/0715 and also by the Research Programme MSM0021620860 of the
Czech Ministry of Education.


\references

Br\'at L. et al., 2008, {\it OEJV}, {\bf 94}, 1

Br\'at L. et al., 2011, {\it OEJV}, {\bf 137}, 1

Burd, A., et al., 2005, {\it NewA}, {\bf 10}, 409

Kwee, K. K., van Woerden, H., 1956, {\it BAN}, {\bf 12}, 327

Maciejewski, G., Karska, A., Niedzielski, A., 2002, {\it IBVS}, 5343, 1

Maciejewski, G., Ligeza, P., Karska, A., 2003, {\it IBVS}, 5400, 1

Nagai, K., 2003, {\it VSOLJ VSB}, No. 40

Nagai, K., 2004, {\it VSOLJ VSB}, No. 42

Nagai, K., 2005, {\it VSOLJ VSB}, No. 43

Nagai, K., 2008, {\it VSOLJ VSB}, No. 46

Nagai, K., 2009, {\it VSOLJ VSB}, No. 48

Nagai, K., 2011, {\it VSOLJ VSB}, No. 51

Perryman, M.A.C. \& ESA, 1997, {\it The HIPPARCOS and TYCHO catalogues}, ESA Publications Division, ESA SP Series 1200 
\BIBCODE{1997ESASP1200.....P}

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

Rucinski, S. et al., 2008, {\it AJ}, {\bf 136}, 586

Yilmaz, M. et al., 2009, {\it IBVS}, 5887, 1

\endreferences


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