\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}
\IBVShead{5886}{4 May 2009}

\IBVStitletl{ASAS~J071829-0336.7:}{ Short-Period End for Contact Binaries Redefined}

\IBVSauth{PRIBULLA, T.$^{1,2}$; VA\v{N}KO, M.$^{1,2}$; HAMB\'ALEK, L.$^{2}$}

\IBVSinst{Astrophysikalisches Institut und Universit\"ats-Sternwarte, Schillerg\"asschen 2-3, D-07740 Jena, Germany, \\ e-mail: pribulla@ta3.sk; vanko@astro.uni-jena.de}

\IBVSinst{Astronomical Institute of the Slovak Academy of Sciences, 05960 Tatransk\'{a} Lomnica, The Slovak Republic, \\ e-mail: lhambalek@ta3.sk}

\SIMBADobj{ASAS J071829-0336.7}
\SIMBADobj{ASAS J113031-0101.9}
\IBVStyp{ EW }
\IBVSkey{eclipsing binaries, photometry}
\IBVSabs{CCD photometry at the Star'a Lesn'a Observatory revealed}
\IBVSabs{that two extremely short-period ASAS variables are genuine}
\IBVSabs{contact binaries. While J071829-0336.7 redefines the short-period}
\IBVSabs{end to P = 0.211249 days, the true orbital period of }
\IBVSabs{J113031-0101.9, P =  0.270961 days, is substantially longer} 
\IBVSabs{than given in the ASAS database. The light curve of the latter} 
\IBVSabs{system indicates mass ratio of about q = 0.15 typical for} 
\IBVSabs{A-type systems.} 

\begintext

\vspace*{-0.3cm}

It is well known that classical W UMa-type binaries have spectral types
A-K and we do not observe a single system with components of M spectral 
type. The spectral types and colors correlate with 
orbital periods due to the main-sequence state of the components, which 
results in sharp cut-off in the number of systems at short orbital periods. The physical reasons
for the observed period cut-off are still not clear (see Rucinski, 1992).

The statistical data on the contact binaries have significantly been 
improved by the The All Sky Automated Survey (ASAS) (Pojmanski 1997, 2004;
Paczynski et al., 2006) by discovery of several hundred new systems in 
the magnitude range 8-13 magnitudes. Analysis of their period distribution 
(Rucinski, 2007) showed that the maximum in the contact-binary numbers occurs 
at about $P = 0.27$ days with definite short-period cut-off at about 
0.215-0.22 days. Rucinski (2007) indicated seven contact-binary candidates 
with $P< 0.22$ days in the ASAS sample and selected GSC~1387-475 as 
the best candidate. The photometric and spectroscopic investigation of 
Rucinski \& Pribulla (2008) confirmed that GSC~1387-475 is genuine contact 
binary; at present being record holder for field systems with $P = 0.2178$ days. 
The only known contact binary having shorter period is V34 in globular cluster 
47 Tuc with $P = 0.2155$ days (Weldrake et al., 2004). 

In our investigation we focused on two other ASAS candidates listed in Table~3 
of Rucinski (2007) observable from northern mid-latitudes: J071829-0336.7\linebreak 
(\mbox{$P_{ASAS} = 0.211249$}~days, $V_{max} = 13.75$), and J113031-0101.9 
($P_{ASAS} = 0.213135$ days, $V_{max} = 13.36$). Both stars are 
fainter than $V = 13$ in the maximum, therefore useful spectroscopic observations 
leading to sound analysis would require 8-10m class telescope because of very short 
orbital periods. While 2MASS 
infrared color of J071829-0336.7, $(J-K) = 0.81$ (K7V) is consistent with extremely short 
orbital period, J113031-0101.9 is too blue, $(J-K) = 0.40$ (G4-5V), to be contact 
binary with the orbital period given in the ASAS database.

Both targets were observed at the Star\'a Lesn\'a Observatory of the Astronomical
Institute of Slovak Academy of Sciences using 50cm Newton telescope equipped with 
SBIG ST10MXE CCD camera (see Pribulla \& Chochol, 2003). Expecting both objects to be mid K-type 
binaries the observations were performed in the $R_C$ and $I_C$ filters only. 
The instrumental magnitudes of the targets have been obtained by the aperture photometry 
using the photometrically calibrated frames (dark frame subtraction and flat field division). 
The differential magnitudes have been left in the instrumental photometric system, very close 
to the Johnson-Cousins system. The part of typical CCD frames in the $I_C$ passband for 
either of the targets showing the variable, comparison and check stars are shown in Figs.~1-2. 
The times of minimum light for both systems determined using Kwee \& van Woerden's method are 
listed in Table~1. The preliminary light-curve (LC) analysis has been performed using code {\it ROCHE}
(see Pribulla, 2004).

\IBVS2fig{7.5cm}{5886-f1.eps}{Field of the eclipsing binary ASAS~J071829-0336.7. The size of the field is 11\farcm20 $\times$ 8\farcm33.}{5886-f2.eps}{Field of the eclipsing binary ASAS~J113031-0101.9. The size of the field is 11\farcm20 $\times$ 8\farcm33.}
\IBVSfigKey{5886-f1.eps}{J071829-0336.7}{finding chart}
\IBVSfigKey{5886-f2.eps}{J113031-0101.9}{finding chart}

{\flushleft{{\bf Table 1.} Times of primary (I) and secondary (II) minima for both eclipsing binaries. 
Weighted averages from individual filters are given.}}

\vskip 3mm
\begin{center}
\begin{tabular}{ccclc}
\hline
Star           & HJD          &  Error & Filters  & Type  \\
\hline
J071829-0336.7 & 2454845.4267 & 0.0002 & $(RI)_C$ & I     \\
               & 2454865.3905 & 0.0001 & $(RI)_C$ & II    \\
               & 2454916.3039 & 0.0001 & $(RI)_C$ & II    \\
J113031-0101.9 & 2454905.4210 & 0.0005 & $R_C$    & I     \\
               & 2454917.4807 & 0.0002 & $(RI)_C$ & I     \\
\hline
\end{tabular}
\end{center}


{\bf J071829-0336.7} Although this target has been observed during 
three nights \linebreak(Jan~13/14, Feb 2/3 and Mar 25/26, 2009) only, thanks to the 
extremely short orbital period, full phase coverage has been achieved. 
The differential LC of J071829-0336.7 has been obtained with 
respect to USNO-A 0825.04506649 having similar color. The stability of this 
comparison has been checked using USNO-A 0825.04514810. Our new photometry 
(Fig.~3) definitely shows that the system is a close binary and not a pulsating 
variable. Very short orbital period given in the ASAS catalogue has been checked
by fitting trigonometric polynomials of the 12th degree to the phase diagrams 
of our $I_C$ data for test periods between 0.205 and 0.215 days. The best period was 
later optimized by non-linear least squares fitting of even trigonometric polynomials 
resulting in the following ephemeris for the primary minimum: 
\begin{equation}
{\rm HJD\ (\mbox{MinI})} =  2\,454\,874.7916(3) + 0.2112594(6) E,
\end{equation} 
compatible with the ASAS result. When scrutinizing the CCD frames of the system a faint 
companion has been noticed west of the system which could not be separated during the 
aperture photometry, but which adds unknown amount of third light. The minima
of the system are partial, therefore geometric parameters cannot reliably be 
determined without spectroscopic mass ratio. The observed LC could be 
solved successfully for a large range of mass ratios between 0.55 and 0.85 (limited by 
rather large photometric amplitude, about 0.55 mag). Assumption of the W-type classification 
(less massive component slightly hotter) always resulted in better $\chi^2$. For the case
of $q$=0.65, convective envelope, temperature of the hotter component 4020 K (K7V), marginal contact 
(fill-out~=~0) the orbital inclination is $i$ = 76\fdeg8. The system sets the new
short-period limit for contact binaries.


\IBVSfig{8.0cm}{5886-f3.eps}{$R_C$ (gray triangles) and $I_C$ (black circles) LCs of J071829-0336.7. }
\IBVSfigKey{5886-f3.eps}{J071829-0336.7}{light curve}


{\bf J113031-0101.9} The LC of the variable (Fig.~4) has been obtained by aperture photometry with 
respect to GSC~4930-00167. The stability of the comparison has been checked using 
USNO-A 0825.07480282. The CCD photometry (Jan 25/26, Feb 2/3, Feb~29/Mar~1, 
Mar 14/15, 25/26, 26/27, 2009) showed that the orbital period given in the ASAS catalogue, 
$P_{ASAS}$ = 0.213135 days, is spurious. This became evident from the observing 
run on March 14/15 which covered both minima and indicated orbital 
period substantially longer, being about 0.270 days. Unfortunately, ASAS photometry
is too noisy to reliably determine/improve the orbital period. Therefore, the orbital period has 
been searched by fitting trigonometric polynomials of the 12th degree 
to the phase diagrams of our $I_C$ data for test periods between 0.25 and 0.29 days. 
The best period was later optimized by non-linear least squares fitting of even 
trigonometric polynomials resulting in the following ephemeris for the minimum:
\begin{equation}
{\rm HJD (\mbox{MinI})}  =  2\,454\,905.2867(3) + 0.270969(4) E.
\end{equation} 

The Star\'a Lesn\'a data (Fig.~4) show that the system is totally eclipsing,
but otherwise rather usual contact binary. Thanks to the total eclipses the 
geometric parameters, \mbox{$q = 0.15$}, $i = 88\deg$, and fill-out $f = 0.5$ 
are rather reliable (unless there was third light). The system, however, 
requires photometric observations from at least 1m telescope. 

\IBVSfig{8.0cm}{5886-f4.eps}{$(RI)_C$ light curves of J113031-0101.9. The centered symbols as in Fig.~3.}
\IBVSfigKey{5886-f4.eps}{J113031-0101.9}{light curve}


\textbf{Acknowledgements} TP and MV acknowledge support from the EU in the 
        FP6 MC ToK project MTKD-CT-2006-042514. This work was supported by 
        GAV Grant no. 2/7010/7 of the Slovak Academy of Sciences.

\references

Paczynski, B., Szczygiel, D., Pilecki, B., Pojmanski, G., 2006, {\it MNRAS}, 
   {\bf 368}, 1311

Pojmanski, G., 1997, {\it Acta Astr.}, {\bf 47}, 467

Pojmanski, G., 2004, {\it Astron. Nachr.}, {\bf 325}, 553

Pribulla, T., 2004, {\it ASPC}, {\bf 318}, 117, in {\it In Spectroscopically 
and Spatially Resolving the Components of the Close Binary Stars},
       R.W. Hilditch et al. (eds.)

Pribulla, T., Chochol, D., 2003, {\it Baltic Astronomy}, {\bf 12}, 555

Rucinski, S.M., 1992, {\it AJ}, {\bf 103}, 960

Rucinski, S.M., 2007, {\it MNRAS}, {\bf 382}, 393

Rucinski, S.M., Pribulla, T., 2008, {\it MNRAS}, {\bf 388}, 1831

Weldrake D.T.F., Sackett P.D., Bridges T.J., Freeman K.C., 2004, {\it AJ}, 
    {\bf 128}, 736

\endreferences

\IBVSedata{5886-t2.txt}
\IBVSdataKey{5886-t2.txt}{ASAS J071829-0336.7}{photometry}
\IBVSedata{5886-t3.txt}
\IBVSdataKey{5886-t3.txt}{ASAS J071829-0336.7}{photometry}
\IBVSedata{5886-t4.txt}
\IBVSdataKey{5886-t4.txt}{ASAS J113031-0101.9}{photometry}
\IBVSedata{5886-t5.txt}
\IBVSdataKey{5886-t5.txt}{ASAS J113031-0101.9}{photometry}

\end{document}
