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\IBVShead{5391}{13 March 2003}
\IBVStitletl{V781 T\lowercase{au}: Improved Evidence for an}{ Orbital Period Change}

\IBVSauth{Donato, L.$^1$; Lepardo, A.$^1$; Santini, V.$^1$; Tomov, T.$^2$; Munari, U.$^3$; Zwitter, T.$^4$}

\IBVSinst{Associazione Friulana di Astronomia e Meteorologia, Piazza Ten.Col. Miani, 33047 Remanzacco (UD), Italy}

\IBVSinst{Centre for Astronomy, N.\ Copernicus University, ul.\ Gagarina 11, 87100 Torun, Poland}

\IBVSinst{Osservatorio Astronomico di Padova - INAF, Sede di Asiago, I-336032 Asiago (VI), Italy}

\IBVSinst{University of Ljubljana, Department of Physics, Jadranska 19, SI-1000 Ljubljana, Slovenia}

\SIMBADobj{V781 Tau}
\IBVStyp{ EA }
\IBVSkey{photometry, period change}

\IBVSabs{V781 Tau (=SAO 077615) is a G0 over-contact binary with an orbital}
\IBVSabs{period that is changing with time. Liu and Yang (2000) proposed a}
\IBVSabs{quadratic orbital ephemeris with the binary period changing}
\IBVSabs{with time. They analysed observations from the literature and added}
\IBVSabs{two minima obtained in 1997 and 1998. These two points proved crucial}
\IBVSabs{for the determination of the parabolic term of the ephemeris and need}
\IBVSabs{robust confirmation by extending measurements of the times of minima}
\IBVSabs{well into the descending branch of the parabolic approximation.}
\IBVSabs{We report here on 10 additional minima secured between Nov-2001 and}
\IBVSabs{Jan-2003 to the aim of confirming the trend and strengthen the}
\IBVSabs{solution. Our results fully confirm the orbital ephemeris of}
\IBVSabs{Liu and Yang.  }

\begintext

V781~Tau ($=$ SAO 077615) is a G0 over-contact ($\sim 23$\%) binary. 
It was discovered by Harris (1979).
Cereda et al.\ (1988) presented a photometric, and Lu (1993)
a radial velocity analysis, while Zwitter et al.\ (2003) explored 
the spectroscopic and photometric solution based on Hipparcos photometry
and GAIA-mode spectroscopic observations.

The orbital period is 0.34 days with the value changing with time.
Liu \&\ Yang (2000) established that the binary ephemeris published 
by Cereda et al.\ (1988):
\begin{equation}
\mathrm{Min.I} = \mathrm{HJD}\ 2443853.9096 + 0.3449094 \times E 
\end{equation}
was not followed exactly. The residuals pointed to a quadratic 
solution with the binary period decreasing with time: 
\begin{equation}
\mathrm{Min.I} = \mathrm{HJD}\ 2443853.9110 + 0.344909292 \times E - 
2.5 \times 10^{-11} \times E^2
\end{equation}
This corresponds to a period decrease of ${\rm d}P/P = -5.0 \times 10^{-11}$. 

Conclusions of Liu \&\ Yang (2000) were based on 14 photographic observations
of minima by Berthold (1983) and on 15 photoelectric minima determinations 
by Cereda et al.\ (1988), Pohl et al.\ (1987) and Keskin \&\ Pohl (1989). 
Liu \&\ Yang added two minima obtained in 1997 and 1998. These two points 
proved crucial for the determination of the parabolic term of the 
ephemeris (Eq.\ 2) and need robust confirmation by extending
measurements of the times of minima well into the descending branch of
the parabolic approximation. We report here on 10 additional
minima secured between Nov. 2001 and Jan. 2003 to the aim of confirming the 
trend and strengthen the solution. This increases the total number 
of minima observed after the year 1990 to 12, therefore substantially 
decreasing the uncertainty of the orbital ephemeris.  

Observations were obtained at the Remanzacco observatory 
(13\deg 18\arcm 59\arcs\ E, 46\deg 5\arcm 11\arcs~N) by members of 
A.F.A.M. (Associazione Friulana di Astronomia e Meteorologia). 
A 0.45-m F/24 Cassegrain telescope was used. The detector was an
1P21 photoelectric photometer. $B$ and $V$ filters conform to the 
Johnson system. Table~1 summarizes the observing log. Each data 
point listed in the N$^{\underline{\mathrm{o}}}$ column of Table 1 is 
actually an average of twelve consecutive 5-sec integrations.  
The comparison star was TYC 1870-514-1 
($V$=9\fmm68, $B$=10\fmm08) with colours very similar to V781~Tau. Typical 
errors of individual $V$ and $B$ band observations of V781~Tau 
are $\sim 0.007$~mag. 

\begin{table}[!ht]
\begin{center}
{\normalsize
{\bf Table 1.}
Observations of V781 Tau used to determine times of photometric minima. 
Columns give the HJD, number of observations and type of filter 
used. 
}
\end{center}
\begin{center}
\begin{tabular}{lrc|lrc|lrc}
\hline
HJD    & N$^{\underline{\mathrm{o}}}$ 
           & filter &HJD    & N$^{\underline{\mathrm{o}}}$ 
	                        & filter &HJD    & N$^{\underline{\mathrm{o}}}$ 
				                     & filter \\ \hline
2452229& 56& $B$      &2452230& 51& $B$      &2452231& 89& $B$      \\ 
2452252& 82& $B$      &2452260& 41& $B$      &2452587& 21& $V$      \\
2452652& 41& $V$      &2452658& 60& $V$      &2452659& 49& $V$      \\
2452665& 86& $B$      &2452666& 61& $B$      &       &   &        \\ \hline  
\end{tabular}
\end{center}
\end{table}

Times of minima were determined by the algorithm of Kwee \&\ Van Woerden (1956).
Table 2 reports times of 10 new minima together with their cycle number 
and residuals with respect to the ephemeris of Equation 1. 

\begin{table}[!ht]
\begin{center}
{\normalsize
{\bf Table 2.}
Times of photometric minima with their standard deviations, type of 
minimum and filter used. The last two columns 
give the cycle number and ($O-C$ in days) according to Equation~1.}
\end{center}
\begin{center}
\begin{tabular}{lllll}
\hline
HJD                   & type & filter & E     & ($O-C$) \\
\hline
2452229.5042 \ $ \pm $ 0.0018 & sec. & $B$ & 24283.5 & $-$ 0.0128 \\
2452230.5361 \ $ \pm $ 0.0009 & sec. & $B$ & 24286.5 & $-$ 0.0157 \\
2452231.3946 \ $ \pm $ 0.0006 & prim.& $B$ & 24289.0 & $-$ 0.0194 \\
2452231.5723 \ $ \pm $ 0.0012 & sec. & $B$ & 24289.5 & $-$ 0.0142 \\
2452252.4369 \ $ \pm $ 0.0066 & prim.& $B$ & 24350.0 & $-$ 0.0166 \\
2452252.6076 \ $ \pm $ 0.0057 & sec. & $B$ & 24350.5 & $-$ 0.0183 \\
2452260.3694 \ $ \pm $ 0.0027 & prim.& $B$ & 24373.0 & $-$ 0.0170 \\
2452658.3967 \ $ \pm $ 0.0066 & prim.& $V$ & 25527.0 & $-$ 0.0151 \\
2452659.4252 \ $ \pm $ 0.0039 & prim.& $V$ & 25530.0 & $-$ 0.0214 \\
2452666.3276 \ $ \pm $ 0.0015 & prim.& $B$ & 25550.0 & $-$ 0.0172 \\ \hline
\end{tabular}
\end{center}
\end{table}

Figure 1 is an $O-C$ diagram of the period change for V781~Tau. 
The figure is identical to that in Liu \&\ Yang (2000), but 
supplemented with our new observations from Table 2. We note 
that our minima show a certain degree of scatter. Average difference between 
the observed points and the parabolic solution is $- 0.0003 \pm 0.0024$ days.
This is in general agreement 
with errors of minima determination (Table 2). But average 
($O-C$) residuals for the two observing seasons lie exactly on the 
parabolic ephemeris given by Equation 2. Our new data points 
clearly confirm the parabolic ephemeris of Liu \&\ Yang. 
The observations from the literature have widely varying and 
sometimes subjective error estimates. We~therefore refrain from 
re-estimation of errors of the Liu \&\ Yang ephemeris. 

We conclude that the period change in V781~Tau is now better constrained. 
Wang~(1994) and Liu \&\ Yang (2000) suggested that period decreases 
due to shrinking of the less massive star in a binary. The stars are in 
contact, so the missing volume is immediately 
filled by material from the other star. Change in the mass ratio 
of the stars finally decreases the orbital period. 
The shrinking star releases some 
gravitational energy to support its surface effective temperature higher 
than the other star. Zwitter et al.\ (2003) find that 
the component with the lower mass in V781~Tau 
is $\sim 220$~K hotter than the more massive one.  
Such a scenario may be common among the binaries of W~UMa type. 

\clearpage

\IBVSfig{12cm}{5391-f1.eps}{
$O-C$ diagram of the period change for V781~Tau. Crosses indicate photographic
observations and plus signs are photoelectric observations from the 
literature. Open circles mark photoelectric minima from this paper. 
Filled circles are their yearly averages for the 2001/2002 and 2002/2003 
observing seasons. The curve denotes parabolic ephemeris from Equation~2.}
%%\IBVSfigKey{5391-f1.eps}{V781 Tau}{periodogram}

We would like to thank M. Dal B\`{o}, L. D'Alessandro, E. Dembitzer,
A. Frigo, S. Garzia, V. Gonano, M. Maestrutti, L. Monzo,  
R. Passuello and G. Sostero for valuable assistance and support.

\references

Berthold, T., 1983, {\it IBVS}, No. 2443

Cereda, L., Mist\`{o}, A., Niarchos, P. G., Poretti, E., 1988, {\it A\&AS}, {\bf 76}, 255

Harris, A. W., 1979, {\it IBVS}, No. 1556

Keskin, V., Pohl, E., 1989, {\it IBVS}, No. 3355

Kwee, K. K., Van Woerden, H., 1956, {\it B.A.N.}, {\bf 12}, 327

Liu, Q., Yang, Y., 2000, {\it A\&AS}, {\bf 142}, 31

Lu, W., 1993, {\it AJ}, {\bf 105}, 646

Pohl, E., Akan, M. C., Ibanoglu, C., Sezer, C., Gudur, N., 1987, {\it IBVS}, No. 3078

Wang, J. M., 1994, {\it ApJ}, {\bf 434}, 277

Zwitter, T., Munari, U., Marrese, P. M., Pr\v{s}a, A., Milone, E. F., 
Boschi, F., Tomov, T., Siviero, A., 2003, {\it A\&A}, submitted 

\endreferences

\end{document}

