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\IBVShead{4435}{6 February 1997}

\centerline{\ft SUDDEN PERIOD CHANGE IN THE CONTACT BINARY AW UMa?}
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\begintext

   The orbital period change of the contact binary AW UMa was firstly reported
by Woodward et al. (1980). Hrivnak (1982) considered it as a sudden decrease
of the period from one constant period (0.43873231 days) to another
one (0.43872917 days), which occurred around 1976 or as a continuous decrease. 
Due to the lack of observations it was impossible to decide, which type of
period change occurred. 

   Our UBV photoelectric observations were carried out at the Star\'a Lesn\'a
Observatory (SL) in 1995-1997, Skalnat\'e Pleso Observatory (SP) in 1992 and 1996
and Kryonerion Station of the National Observatory of Athens (K) in 1982 and 1986.
The telescopes and their equipments are described in Hric et al. (1991).
BD~+31\degr 2270 was used as the comparison star. U, B, V light curves of
AW UMa based on the 1995 and 1996 data are depicted in Figure 1. Mid-eclipse
brightening was registered in the secondary minimum (Pribulla and Chochol,
1997). Derman et al. (1990) and Bakos et al. (1991) reported pronounced
light and colour variations of AW UMa in 1989-90. Our light curves show
that AW UMa is in a quiet phase now. The times of minima and their standard
errors, determined using Kwee and van Woerden's (1956) method are given in
Table 1.
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\begin{center}
Figure 1. U, B and V light curves obtained at Star\'a Lesn\'a Observatory
in 1995 and 1996. Phases were calculated using the ephemeris (2).
\end{center}

\vfill \eject

\centerline{2}
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\centerline{Table 1. Times of minima of AW UMa}
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\begin{center}
\begin{tabular}{|lcccc|lcccc|}
\hline
JD$_{hel}$& $\sigma$ &  Min.& Obs.&Filt.&JD$_{hel}$& $\sigma$ &  Min.  & Obs.&Filt. \\
2400000+  &$\times 10^{-4}$&  &  &  &2400000+  & $\times 10^{-4}$&    & &\\
\hline
45107.4766 &   2    &   I  & K  &BV &50139.4693 &  2    &   II   & SL&UBV   \\
45108.3531 &   4    &   I  & K  &BV &50141.4447 &  3    &   I    & SL&UBV  \\
46514.4813 &  10    &   I  & K  &BV &50161.4076 &  3    &   II   & SL&UBV  \\
46515.3586 &   3    &   I  & K  &BV &50421.571  &  10   &   II   & SP&UBVR \\
48683.5554 &   5    &   I  & SP &V  &50423.5438 &  4    &   I    & SL&UBV  \\
49778.3977 &   2    &   II & SL &UBV&50428.5918 &  1    &   II   & SL&UBV  \\
49862.4120 &   5    &   I  & SL &UBV&50430.5640 &  3    &   I    & SP&UBV  \\
50096.478  &  10    &   II & SL &UBV&50461.4979 &  4    &   II   & SL&UBV  \\
50097.5706 &  2     &   I  & SL &UBV&50465.4433 &  1.5  &   II   & SL&B    \\
50098.4489 &  0.2   &   I  & SL &UBV&50471.5855 &  1    &   II   & SL&V    \\
\hline
\end{tabular}
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\centerline{\epsffile{4435-f2.ps}}
\centerline{Figure 2. The O$-$C diagram}
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The times of minima given in Table 1 together with the data published by
Yim and Jeong (1995) and M\"uyessero\v{g}lu et al. (1996)  as well as the data
compiled from literature by Bakos et al. (1991) and Demircan et al. (1992)
were used to study period change. The O$-$C residuals (Figure 2) were
calculated using the ephemeris:
\begin{equation}
{\rm Min~I} = HJD\,2\,438\,044.8164\,+\,0\fd 43872901\times~E.
\end{equation}

As it is apparent from Figure 2, the data could be explained either by two sudden
period changes, which occurred in 1976 and 1994 or by a continuous period change.
The linear ephemeris between the two sudden period changes (1975-1994) is
identical with ephemeris (1). The period 0.43873231 days determined by Hrivnak
(1982) and our ephemeris (1) indicate a period jump $\Delta P/P = 7.5\times10^{-6}$.
The minima after the second jump are defined by the following linear
ephemeris:

\begin{equation}
{\rm Min\,I = HJD}\,2\,438\,044.8625 + 0\fd 43872703 \times E
\end{equation}
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\centerline{3}
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The corresponding period jump ($\Delta P/P = 4.4\times10^{-6}$) is smaller
than the first one.
The data in Figure 2 fitted by a parabola (continuous period change)
are represented by the following ephemeris:

\begin{equation}
{\rm Min\,I = HJD}\,2\,438\,044.7824 + 0\fd 43873301 \times E - 1.105~10^{-10} \times E^2
\end{equation}

The sum of squares of the residuals for the three linear fits
(2.5~10$^{-5}d^{2}$) is half of that for the quadratic fit
(5.0~$10^{-5}d^{2}$), therefore the sudden period change seems to be more 
probable than the continuous one.

\indent {\bf Acknowledgements.} This work was partly supported by a NATO
grant (No.\,960322) and by a VEGA grant 1282/1997. 
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\beginauthors
T. PRIBULLA
D. CHOCHOL
Astronomical Institute
Slovak Academy of Sciences
059 60 Tatransk\'a Lomnica
Slovakia
e-mail: pribula@auriga.ta3.sk
\vspace{1cm}
P. ROVITHIS
Astronomical Institute,
The National Obs. of Athens,
P.O. Box 20048,
GR 11810 Athens,
Greece
\authorsrightcol
H. ROVITHIS-LIVANIOU
Section of Astrophysics,
University of Athens,
GR 15784 Zografos,
Greece
e-mail: elivan@atlas.uoa.gr
\vskip 45mm
\endauthors


\references

Bakos, G.A., Hor\'ak, T.B., Tremko, J., 1991, {\it Bull. Astron. Inst. Czechosl.}, {\bf 42}, 331

Demircan, O., Derman, E., M\"uyessero\v{g}lu, Z., 1992, {\it A\&A }, {\bf 263}, 165

Derman, E, Demircan, O., M\"uyessero\v{g}lu, Z., 1990, {\it IBVS}, No.\,3540

Hric, L., Skopal, A., Urban, Z., 1991, {\it Contrib. Astron. Obs. Skalnat\'e Pleso}, {\bf 21}, 303

Hrivnak, B.J., 1982, {\it Astrophys. Journal}, {\bf 260}, 744

Kwee, K.K. \& van Woerden, H., 1956, {\it Bull. Astron. Inst. Neth.}, {\bf 12}, 327

M\"uyessero\v{g}lu, Z., G\"urol, B., Selam, S.O., 1996, {\it IBVS}, No.\,4380

Pribulla, T., Chochol, D., 1997, {\it In: Proceedings of the conference
``Contemporary problems of astronomy" }, Odessa, in press

Woodward, E.J., Koch, R.H., Eisenhardt, P.R., 1980, {\it Astron. Journal}, {\bf 85}, No.\,1, 50

Yim, J.R., Jeong, J.H., 1995, {\it J. Astron. Space Sci.}, {\bf 12}, 44


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