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\IBVShead{4294}{7 February 1996}             % Use it with dummy or empty a

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\title{PHOTOMETRY OF THE ACTIVE STAR UZ LIBRAE}

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We present new $UBV$ photometry for the RS CVn binary UZ Librae
(BD $-$08\degr 3999, $V$ = 9.3 mag). This star is one of the most
active binary systems of the RS CVn class. In the past UZ Librae showed
photometric variations with an amplitude of up to 0.35 mag in $V$.
Evans \& Bopp (1974) suggested that UZ Lib was most probable a spotted
flare star. Further photometry of UZ Librae was presented by Heckert \&
Hickman (1991) from 1988, 1989 and by Heckert (1992, 1993) from 1990 to 1992.
Its photospheric line-profile variability was first detected by Bopp et al.
(1984) confirming that the stellar surface is indeed covered by large
starspots.
Grewing et al. (1989) discovered the -- optically unseen -- companion star
in the ultraviolet spectrum ($T_{\rm eff}\approx$ 8000\,K and $R\approx$
1$R_\odot$). They also concluded that the binary-system properties fit
well the evolutionary scenario of a coalescing binary with almost bound
rotation. Table 1 summarizes the stellar parameters for UZ Librae from the
papers by Grewing et al. (1989) and Strassmeier (1996).

\begin{center}
\begin{tabular}{ll}
\hline
Parameter & Adopted \\
\hline
Spectral type & K0III \\
log $g$ & 2.5 \\
$T_{\rm eff}$ & 4800 K \\
$v\sin i$ & 67 km\,s$^{-1}$ \\
Inclination $i$ & 30\degr \\
Rotation period & 4.73574 days \\
Orbital period & 4.76864 days \\
\hline
\end{tabular}
\end{center}

Our new observations were made with the 50cm ESO-telescope at ESO, La Silla
(observer E. Paunzen) in the nights between July 22nd and July 30th, 1994.
We chose BD\,~$-$07\degr 4044 as the comparison star and BD\,$-$08\degr 3998 as the
check star. All measurements were transformed to the Johnson {\em UBV\/}
system. Integration times of 40 seconds were used for $B$ and $V$,
respectively and 90 seconds for $U$. All data were corrected for sky
background and extinction. Phases were computed with the photometric
ephemeris of Grewing et al. (1989)
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T[$\phi$=0] = HJD 2445426.122 + 4.73574$\times$E . \\
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Figure 1 shows the differential data  in the sense UZ Librae $-$
BD $-$07\degr 4044) in $V$, $(B-V)$ and $(U-B)$. The data of the comparison
minus check star show no variability above the uncertainties and
we conclude that both are constant as reported before.

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Figure 1.
$UBV$ light and color curves for UZ Librae. The data are plotted
with the photometric ephemeris T[$\phi$=0] = HJD 2445426.122 + 4.73574$\times$E
\end{center}
\large
\vskip 4mm

We used a standard period-finding program to confirm the four-day period;
the formal value from our small data set is 4.9$\pm$0.2 days. The full
amplitude in $V$ is 0.16$\pm$0.01 mag, and 0.06$\pm$0.02 mag and
0.10$\pm$0.02 mag in $(B-V)$ and $(U-B)$, respectively. We note that the
$(U-B)$ color curve is in phase with the $V$-light curve, as is the $(B-V)$
color curve, and shows a well-defined and large amplitude as already
indicated by Heckert (1992).
Compared to previous observations, however, UZ Librae seems to be
in a low-amplitude state but maintained its ``two-spot'' character. 

{\it Acknowledgement}: Work on stellar activity at the University of Vienna is
supported by grants FWF-S7301 and \"OAW-OWP40 to KGS and by the
Hochschuljubil\"aumsstiftung der Stadt Wien to EP.


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\centerline{3}
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\centerline{Table 1. Johnson $UBV$ photometry for UZ Librae}
\begin{center}
\begin{tabular}{|c|c|c|c|c|c|}
\hline
\hline
\multicolumn{6}{|c|}{UZ Librae $-$ BD $-$07\degr 4044} \\
\hline
HJD\,2440000+ & $\Delta V$ & HJD\,2440000+ & $\Delta (U-B)$ & HJD\,2440000+
& $\Delta (B-V)$ \\
\hline
9556.47355&   $-$0.300& 9556.47256&   $-$0.280& 9556.47306&   $-$0.050\\
9556.47506&   $-$0.290& 9556.47405&   $-$0.290& 9556.47455&   $-$0.073\\
9556.47654&   $-$0.308& 9556.47554&   $-$0.283& 9556.47604&   $-$0.073\\
9557.48613&   $-$0.403& 9557.48516&   $-$0.338& 9557.48565&   $-$0.070\\
9557.48822&   $-$0.410& 9557.48722&   $-$0.338& 9557.48772&   $-$0.068\\
9557.49029&   $-$0.400& 9557.48929&   $-$0.340& 9557.48979&   $-$0.073\\
9558.47230&   $-$0.275& 9558.47130&   $-$0.250& 9558.47179&   $-$0.043\\
9558.47438&   $-$0.278& 9558.47338&   $-$0.240& 9558.47388&   $-$0.048\\
9558.47644&   $-$0.283& 9558.47546&   $-$0.242& 9558.47595&   $-$0.037\\
9559.47554&   $-$0.280& 9559.47453&   $-$0.313& 9559.47502&   $-$0.050\\
9559.47762&   $-$0.278& 9559.47662&   $-$0.310& 9559.47712&   $-$0.053\\
9559.47968&   $-$0.260& 9559.47869&   $-$0.295& 9559.47712&   $-$0.048\\
9560.47589&   $-$0.248& 9560.47492&   $-$0.258& 9560.47541&   $-$0.037\\
9560.47796&   $-$0.250& 9560.47699&   $-$0.265& 9560.47748&   $-$0.037\\
9560.48003&   $-$0.245& 9560.47904&   $-$0.250& 9560.47953&   $-$0.045\\
9561.48771&   $-$0.340& 9561.48670&   $-$0.290& 9561.48720&   $-$0.063\\
9561.48976&   $-$0.343& 9561.48877&   $-$0.288& 9561.48926&   $-$0.057\\
9561.49182&   $-$0.343& 9561.49082&   $-$0.285& 9561.49132&   $-$0.057\\
9562.49892&   $-$0.373& 9562.49814&   $-$0.297& 9562.49853&   $-$0.057\\
9562.50039&   $-$0.375& 9562.49965&   $-$0.295& 9562.50001&   $-$0.057\\
9563.58574&   $-$0.273& 9563.58499&   $-$0.280& 9563.58537&   $-$0.048\\
9563.58722&   $-$0.265& 9563.58647&   $-$0.278& 9563.58685&   $-$0.035\\
9564.54091&   $-$0.293& 9564.53990&   $-$0.300& 9564.54041&   $-$0.060\\
9564.54292&   $-$0.288& 9564.54200&   $-$0.295& 9564.54240&   $-$0.053\\
9564.54497&   $-$0.285& 9564.54398&   $-$0.293& 9564.54448&   $-$0.048\\
\hline
\hline
\end{tabular}
\end{center}
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\beginauthors
\authorsrightcol
E. PAUNZEN
K. G. STRASSMEIER
Institut f\"ur Astronomie, 
T\"urkenschanzstra\ss e 17, A-1180 Wien,
Austria
e-mail: paunzen@astro.ast.univie.ac.at
e-mail: strassmeier@astro.ast.univie.ac.at
\endauthors
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\references

Bopp, B.W., Goodrich, B.D., Africano, J.L., Noah, P.V., Meredith, R.J., Palmer, L.H.,
Quigley, R.J., 1984, {\it ApJ}, {\bf 285}, 202

Evans, D.F., Bopp, B.W., 1974, {\it Observatory}, {\bf 94}, 80

Grewing, M., Bianchi, L., Garrido, R., 1989, {\it A\&A}, {\bf 223}, 173

Heckert, P.A., Hickman, M.A., 1991, {\it IBVS}, No.\,3570

Heckert, P.A., 1992, {\it IBVS}, No.\,3796

Heckert, P.A., 1993, {\it IBVS}, No.\,3953

Strassmeier, K.G., 1996, {\it A\&A}, submitted

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