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\begin{document}
\IBVShead{4681}{10 March 1999}

\IBVStitletl{New Times of Minima and Updated Ephemerides}{ of Selected Contact Binaries}

\IBVSauth{L.L.~Kiss$^1$, G.~Kasz\'{a}s$^2$, G.~F\H ur\'esz$^1$, J.~Vink\'{o}$^2$}

\IBVSinst{Dept.\ of Exp.\ Phys., JATE University,
D\'{o}m t\'{e}r 9., H-6720 Szeged, Hungary,
e-mail: l.kiss@physx.u-szeged.hu}

\IBVSinst{Dept.\ of Optics, JATE University,
H-6701 Szeged, P.O.Box 406, Hungary,
e-mail: kaszas@physx.u-szeged.hu}

\IBVSkey{photometry}
\IBVSabs{We present new times of minima of 44i~Boo, AB~And, SW~Lac, U~Peg}
\IBVSabs{VW~Cep and XY~Leo. Besides these stars, we also give updated }
\IBVSabs{ephemerides of OO~Aql, DK~Cyg, LS~Del and V1073~Cyg.}

\begintext
The period variation of contact binary systems of the W~UMa-type 
is a controversial issue of binary star astrophysics. It is known
that W~UMa-stars show a wide variety of period change (Kreiner, 1977),
but the cause of the long- as well as short-term variations is
still a mystery (see e.g.\ Kalimeris {\it et al.}, 1994 for a discussion
of possible physical mechanisms). There are pieces of evidence that
the observed period variations of contact binaries are basically 
random phenomena (van't Veer, 1991; Rucinski, 1993), besides the 
occasional light-time effect caused by a third component. 
On the other hand, it was pointed
out very recently by Kasz\'as {\it et al.}\ (1998) that the period
variation of VW~Cep seems to be also affected by the magnetic 
activity cycle.

The aim of this paper is twofold: first, to present new times of
minima of bright W~UMa-systems in order to contribute to the
period variation studies, and second, to provide up-to-date accurate 
ephemerides of a sample of W~UMas that can be used to predict
orbital phases in the present and the next few observing seasons. 
Such ephemerides are essential to perform high-resolution 
spectroscopic studies, where the accurate phase of the system that
can be assigned to the observed spectrum must be precisely known. 

\begin{figure}
\begin{center}
\input 4681-f1.fig

{\figfon Figure 1. \normalsize Light- and colour curve of SW~Lac}
\end{center}
\end{figure}

We obtained Johnson BV photometric measurements of the W~UMa-stars 
listed in the first column of Table~1, with the 40 cm Cassegrain 
telescope at Szeged Observatory between 1995 and 1998. A sample
light- and colour curve of SW~Lac illustrating the photometric accuracy
is plotted in Fig.~1 as a function of phase.
The times of minima calculated
by low-order polynomial fitting to the data points close to 
the particular minimum are collected in the second column of 
Table~1. The typical uncertainty of these moments is 
$\pm 0.0005$ day.

We have also collected recently published times of minima for a sample of
bright W~UMa-stars that were selected for targets of our spectroscopic
observing program. The spectroscopic results will be published elsewhere.
Here we present the updated ephemerides of these program stars that can be
used for calculating phases accurate to $\pm 0.01$ during the next observing
season (provided the systems do not exhibit sudden and violent period
changes in the meantime).

\begin{table}
\caption{New times of minima of W~UMa systems}
\begin{center}
\begin{tabular}{cccccc}
\hline
Star & Min (Hel.JD) & type & Star & Min (Hel.JD) & type \\
     & 2\,400\,000+ &      &      & 2\,400\,000+ &      \\
\hline
44i Boo & 49828.5515 & II & U Peg & 50728.3701 & II \\
        & 49830.5611 & I &        & 50728.5567 & I \\
        & 49841.4052 & II &       & 50750.2953 & I \\
        & 49841.5414 & I &        &                \\
        & 50180.4658 & II & VW Cep& 50707.2843 & II \\
        & 50182.4747 & I &        & 50707.4248 & I \\
        & 50941.3394 & II &       &            &   \\
        & 50942.4089 & II & XY Leo& 49828.3876 & II \\
        & 50942.5450 & I &        & 50195.3114 & I \\
	& 	     &   &        & 50196.3080 & II \\
AB And  & 50702.3634 & II &       & 50508.3982 & I \\
        & 50702.5283 & I &        & 50513.3698 & II \\
	& 	     &   &        & 50519.3377 & II \\
SW Lac  & 50694.3642 & II &       & 50547.3229 & I \\
        & 50700.2965 & I &        & 50862.3933 & I \\
        & 51035.4430 & I &        & 50862.5359 & II \\
\hline
\end{tabular}
\end{center}
\end{table}

W~UMa-stars usually change their period slowly but continuously over time,
and it can seldom be described as a linear period variation. Since the
purpose of our study was to find approximate periods for a limited duration,
we restricted the time base of our study only to the past five years. {\it
O$-$C} diagrams were computed for all program stars using the data published
recently in {\it IBVS} and the ephemerides in {\it GCVS}. In all cases the
{\it O$-$C} diagrams could be well approximated with a straight line, and
linear least-squares fit provided the correction of the period and the epoch
as the fitted slope and the zero-point, respectively. An example of the
computed {\it O$-$C} diagrams and the fitted line is plotted in Fig~2 in
order to illustrate the validity of the linear approximation of the {\it
O$-$C} diagram over the studied time interval. The list of individual
moments of minima and their references for all stars can be requested from
the first author of this paper via e-mail.

\begin{figure}
\begin{center}
\input 4681-f2.fig

{\figfon Figure 2. \normalsize The {\it O$-$C} diagram of SW~Lac as a function of the cycle number 
{\it E}}
\end{center}
\end{figure}

\begin{table}
\caption{New ephemerides of selected W~UMa-stars.}
\begin{center}
\begin{tabular}{ccccc}
\hline
Star & T Min I              & Period & $\Delta P$ & $\Delta P / \Delta T$ \\
     &  (HJD 2\,400\,000+)  & (days) & ($10^{-6}$ day) & ($10^{-10}$) \\ 
\hline
AB~And & 51000.2368 & 0.33189103 & $-$1.11784 & $-$0.752 \\
OO~Aql & 51000.1473 & 0.50678964 & $+$1.16731 & $+$0.936 \\
VW~Cep & 51000.2054 & 0.27830821 & $-$6.38711 & $-$9.339 \\
DK~Cyg & 51000.0999 & 0.47069290 & $+$2.35766 & $+$1.808 \\
V1073~Cyg & 51000.2912 & 0.78585079 & $-$8.90802 & $-$7.228 \\
LS~Del & 51000.2257 & 0.36384021 & $+$3.41065 & $+$5.826 \\
SW~Lac & 51000.1656 & 0.32071552 & $-$5.37688 & $-$9.398 \\
U~Peg  & 51000.2713 & 0.37477746 & $-$3.97695 & $-$2.740 \\
\hline
\end{tabular}
\end{center}
\end{table}

Table~2 presents the newly inferred ephemerides together with the $\Delta P$
differences (in $10^{-6}$ day) between the periods presented in this paper
and those listed in {\it GCVS}, and the approximate rate of the period
change $\Delta P / \Delta T$ (in $10^{-10}$ day/day) where $\Delta T$ is the
difference between the current epochs and those in {\it GCVS}.
$\Delta P$ may give some hints about usefulness of
older periods in calculating orbital phases, while the other
parameter is a rough measure of the stability of the period over 
a few thousand days, although this is obviously not a proper
representation of the actual period variation, which would require
much longer samples and correct treatment of non-linear terms in
the fitting. 

It is apparent from Table~2 that both the increase and the decrease 
of the period can be observed among the W~UMa-stars analyzed here.
This is a well-known property of contact binaries and it can be confirmed 
by larger statistical samples (e.g.\ Kreiner, 1977). If one assumes 
that these systems remain in stable contact configuration and
the total mass of the system is constant then 
the increase of the period corresponds to a mass transfer from
the less massive to the more massive component (thus, decreasing
the mass ratio), while in the case of period decrease the direction
of the mass transfer is reversed (i.e.\ the mass ratio is increasing).
The thermal-relaxation oscillation theory of contact binaries 
(Lucy \& Wilson, 1979) proposes such kind of mass transfers as
the system is oscillating around the marginal contact state being
temporarily in contact and non-contact phases.
In this picture the systems with increasing period 
(OO~Aql, DK~Cyg, LS~Del) are before the broken-contact phase, 
while the systems with decreasing period (AB~And, VW~Cep, U~Peg,
SW~Lac, V1073~Cyg) are after that.   
 
Concerning the speed of the observed period variations, it can be
seen in Table~2 that the period variation rates are about the same
order of magnitude, $10^{-9}$--$10^{-10}$ day/day for all systems 
studied in this paper. 
Model calculations by Mochnacki (1981) give about $\tau \approx
10^{9}$ years as the lifetime of contact binaries due to magnetic braking. 
A rough estimate of the theoretical period variation rate of the contact
binary is $1 / \tau$ day/day, therefore, the period
variation rates in Table~2 are at least an order of magnitude higher than
the prediction by the magnetic braking mechanism. 
However, more precise treatment of much more extensive
observational data are necessary to address these questions in a
satisfactory and more detailed way.

\bigskip
This research has been supported by OTKA Grants F022249, T022259, 
Soros Foundation Grant 222/2/3624, Szeged Observatory Foundation
and Foundation for Hungarian Education and Science (AMFK). Useful
discussions with Dr.\ K. Szatm\'ary are gratefully acknowledged.

\references

Kalimeris, A., Rovithis-Livaniou, H., Rovithis, P. 
   1994, {\it A\&A}  {\bf 282}, 775

Kasz\'as, G., Vink\'o, J., Szatm\'ary, K., Heged\"us, T., G\'al, J.,
   Kiss, L.L., Borkovits, T. 1998, {\it A\&A}  {\bf 331}, 231

Kreiner, J.M. 1977, in: {\it The Interaction of Variable Stars with
   Their Environment} Proc.\ IAU Coll.\ No.\,42. (ed.\ R. Kippenhahn et al.),
   Bamberg, p. 393 

Lucy, L.B., Wilson, R.E. 1979, {\it ApJ} {\bf 231}, 502
   
Mochnacki, S.W. 1981, {\it ApJ} {\bf 245}, 650   

Rucinski, S.M. 1993, in: {\it The Realm of Interacting Binaries}
  (ed.\ J. Sahade et al.), Kluwer Acad.\ Publ., p.\ 111 

van't Veer, F. 1991, {\it A\&A} {\bf 250}, 84

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