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{\bf Long-term monitoring of active stars. \\ 
VII. UBV(RI)$_c$ photometry collected in March 1991}
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{\bf G.~Cutispoto} 
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{\it Osservatorio Astrofisico di Catania, V.le A.Doria 6, 
I-95125 Catania, Italy; \\
e-mail:~~gcutispoto@alpha4.ct.astro.it}
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~\\
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{\bf abstract} \\
In the framework of an extensive program focusing on the global properties and 
evolution of active stars, high-precision {\it UBV(RI)$_c$} photometry of 19 
selected stars, collected at the European Southern Observatory over the 
15-31 March 1991 interval is presented. Significant evolution of the light 
curves, period variations and evidence for long-term variability of the global 
degree of spottedness are found. Most of the spectral classifications are 
discussed. A flare event was detected for the star HD 127535 = V~841 Cen. 
These observations 
contribute to the establishment of a time-extended photometric database which 
can give important clues on topics such as the stability of spotted areas, 
differential rotation, solar-like cycles and the correlation between 
inhomogeneities at different atmospheric levels.

\newpage
\begin{table}
\caption{Comparison (c) and check (ck) stars for each program star (v) and
standard deviations ($\sigma$) for the v-c and ck-c 
{\it V}-band differential magnitudes for each series of N nights}
\begin{tabular}{|ll|l|l|c|c|r|}
\hline
\multicolumn{2}{|c}{Program star} & \multicolumn{1}{|c}{(c)} & 
\multicolumn{1}{|c|}{(ck)} & $\sigma$ & $\sigma$ & 
\multicolumn{1}{c|}{N} \\
\multicolumn{2}{|c|}{(v)} & & & (v-c) & (ck-c) & \\
\hline
HD 32918  & = YY Men   & HD 33763  & HD 33747   & 54 & 4 & 13 \\
HD 36705  & = AB Dor   & HD 35230  & HD 36316   & 17 & 6 & 11 \\
HD 37847  & = TW Lep   & HD 37653  & SAO 170680 & 41 & 4 & 16 \\
HD 39917  & = SZ Pic   & HD 39901  & HD 39962   & 38 & 5 & 13 \\
HD 61245  & = V344 Pup & HD 61390  & HD 61333   & 22 & 5 & 12 \\
HD 81410  & = IL Hya   & HD 81904  & HD 80991   & 46 & 4 & 14 \\
HD 82558  & = LQ Hya   & HD 82508  & HD 82447   & 25 & 4 & 12 \\
HD 86005  &            & HD 86034  & HD 85849   &  4 & 2 & 10 \\
HD 98712  & = SZ Crt   & HD 97977  & HD 98251   & 20 & 4 & 11 \\
HD 101309 & = V829 Cen & HD 101679 & HD 101215  & 50 & 4 & 13 \\
HD 102077 & = V838 Cen & HD 102076 & HD 102202  & 16 & 4 & 13 \\
HD 106225 & = HU Vir   & HD 106270 & HD 105796  & 72 & 4 & 16 \\
HD 119285 & = V851 Cen & HD 119164 & HD 119076  & 14 & 2 & 13 \\
HD 127535 & = V841 Cen & HD 128227 & HD 128618  & 33 & 4 & 17 \\
HD 136905 & = GX Lib   & HD 136480 & HD 137241  & 49 & 4 & 14 \\
HD 139084 & = V343 Nor & HD 139070 & HD 138363  & 60 & 4 & 13 \\
HD 155555 & = V824 Ara & HD 157427 & HD 154775  & 25 & 7 & 16 \\
HD 174429 & = PZ Tel   & HD 173560 & HD 176557  & 21 & 5 & 17 \\
HD 197481 & = AU Mic   & HD 197237 & HD 197339  & 15 & 8 &  9 \\
\hline
\end{tabular}
\end{table}


\begin{table}
\caption{Mean atmospheric extinction coefficients for La Silla site, 
obtained over the 15-31 Mar 1991 period}
\begin{tabular}{|c|c|c|c|c|c|}
\hline
            &  U  &  B  &  V  &  R  &  I  \\
\hline
Mag/airmass &.520 &.274 &.157 &.107 &.053 \\
\hline
\end{tabular}
\end{table}

\newpage
\begin{table}
\caption{{\it V} magnitude and colors for the c and ck stars derived from
standard stars. Errors are of the order of 0.01 magnitudes. The symbol ":" 
denotes errors of the order of 0.02 magnitudes}
\renewcommand{\arraystretch}{.95}
\begin{tabular}{|l|c|l|c|c|c|}
\hline
\multicolumn{1}{|c|}{c or ck} & {\it V} & 
\multicolumn{1}{|c|}{{\it U-B}} & {\it B-V} &{\it V-R}$_c$ &{\it V-I}$_c$~ \\
\hline
HD 33747  & 8.76 & 0.72 & 0.98 & 0.53 & 1.03 \\
HD 33763  & 8.37 & 0.50 & 0.89 & 0.49 & 0.95 \\
HD 35230  & 7.58 & 0.47 & 0.88 & 0.48 & 0.93 \\
HD 36316  & 7.95 & 1.72 & 1.46 & 0.79 & 1.50 \\
HD 37653  & 8.24 & 0.50 & 0.89 & 0.48 & 0.93 \\
SAO 170680& 8.97 & 0.91 & 1.04 & 0.54 & 1.01 \\
HD 39901  & 6.54 & 1.63 & 1.37 & 0.71 & 1.32 \\
HD 39962  & 7.97 & -.06:& 0.40 & 0.26 & 0.50 \\
HD 61333  & 7.18 & -.64:& -.11 & -.05 & -.09 \\
HD 61390  & 7.92 & 0.70 & 1.01 & 0.52 & 1.03 \\
HD 81904  & 8.02 & 0.72:& 0.98 & 0.50 & 0.98 \\
HD 80991  & 8.51 & 0.83:& 1.04 & 0.52 & 1.02 \\
HD 82477  & 6.11 & 1.18 & 1.18 & 0.61 & 1.17 \\
HD 82508  & 7.58 & 0.35 & 0.71 & 0.40 & 0.80 \\
HD 85849  & 8.52 & 1.07 & 1.14 & 0.58 & 1.12 \\
HD 86034  & 7.89 & 1.32 & 1.26 & 0.63 & 1.20 \\
HD 97977  & 8.85 & 1.66:& 1.42 & 0.75 & 1.42 \\
HD 98251  & 9.21 & 1.47:& 1.31 & 0.69 & 1.28 \\
HD 101215 & 7.79 & 0.07 & 0.15 & 0.10 & 0.20 \\
HD 101679 & 8.12 & 0.79 & 1.09 & 0.59 & 1.15 \\
HD 102076 & 7.10 & 0.75 & 1.00 & 0.51 & 0.98 \\
HD 102202 & 8.86 & 0.19 & 0.67 & 0.38 & 0.73 \\
HD 105796 & 8.06 & 0.93 & 1.06 & 0.55 & 1.04 \\
HD 106270 & 7.58 & 0.29 & 0.72 & 0.40 & 0.77 \\
HD 119076 & 6.87 & 0.88 & 1.15 & 0.60 & 1.15 \\
HD 119164 & 7.20 & 1.14 & 1.29 & 0.64 & 1.22 \\
HD 128227 & 8.33 & 0.82 & 1.07 & 0.57 & 1.10 \\
HD 128618 & 8.03 & 1.63:& 1.46 & 0.77 & 1.47 \\
HD 136480 & 7.35 & 1.12:& 1.16 & 0.60 & 1.15 \\
HD 137241 & 7.36 & 1.10:& 1.14 & 0.59 & 1.11 \\
HD 138363 & 7.12 & 0.95 & 1.14 & 0.61 & 1.15 \\
HD 139070 & 8.71 & 0.80 & 1.12 & 0.60 & 1.16 \\
HD 154775 & 7.60 & 1.97 & 1.59 & 0.87 & 1.75 \\
HD 157427 & 7.41 & 1.69 & 1.50 & 0.81 & 1.55 \\
HD 173560 & 8.72 & 0.11:& 0.64 & 0.36 & 0.70 \\
HD 176557 & 7.19 & 1.76:& 1.48 & 0.78 & 1.47 \\
HD 197237 & 9.02 & 0.83:& 0.98 & 0.51 & 0.96 \\
HD 197339 & 7.39 & 1.44:& 1.29:& 0.66 & 1.23 \\
\hline
\end{tabular}
\end{table}

\newpage
\begin{table}
\caption{Maximum luminosity ({\it V$_{max}$}) and corresponding colors for 
the program stars and inferred spectral classification (Spectral Type)}
\renewcommand{\arraystretch}{1.05}
\begin{tabular}{|ll|r|l|l|l|l|l|}
\hline
\multicolumn{2}{|c|}{Program Star} & \multicolumn{1}{|c|}{${\it V}_{max}$} & 
\multicolumn{1}{|c|}{{\it U-B~}} & \multicolumn{1}{|c|}{{\it B-V~}} & 
\multicolumn{1}{|c|}{{\it V-R}$_c$} & \multicolumn{1}{|c|}{{\it V-I}$_c$~} & 
\multicolumn{1}{c|}{Spectral Type} \\
\hline
HD 32918   & = YY Men   & 7.98 & 0.67 & 1.04 & 0.59 & 1.14 & K1~III \\
HD 36705   & = AB Dor   & 6.89 & 0.37 & 0.83 & 0.49 & 0.97 & K0~IV/V (PMS) \\
HD 37847   & = TW Lep   & 7.35 & 0.76 & 1.09 & 0.60 & 1.16 & K2/3~III + F6~IV \\
HD 39917   & = SZ Pic   & 7.85 & 0.31 & 0.81 & 0.47 & 0.91 & K0/1~IV + G5~IV \\
HD 61245   & = V344 Pup & 6.85 & 0.80 & 1.04 & 0.55 & 1.05 & K1~III \\
HD 81410   & = IL Hya   & 7.33 & 0.74 & 1.03 & 0.55 & 1.09 & K2~IV  \\
HD 82558   & = LQ Hya   & 7.81 & 0.56 & 0.91 & 0.52 & 1.02 & K2~V   \\
HD 86005   &            & 7.18 & 1.01 & 1.31 & 0.69 & 1.34 & K4~III + K1/2~III\\
HD 98712 AB& = SZ Crt   & 8.60 & 1.21 & 1.33 & 0.86 & 1.65 & K7~V + M3~V \\
HD 98712 A &            & 8.74 &      & 1.30 & 0.83 & 1.56 & K7~V   \\ 
HD 98712 B &            &11.25 &      & 1.6  & 1.1  & 2.4  & M3~V   \\
HD 101309  & = V829 Cen & 7.82 & 0.49 & 0.93 & 0.53 & 1.04 & K1/2~IV + G5~V \\
HD 102077  & = V838 Cen & 8.92 & 0.53 & 0.91 & 0.54 & 1.08 & K1/2~V + K2~V  \\
HD 106225  & = HU Vir   & 8.59 & 0.58 & 0.98 & 0.57 & 1.13 & K1~IV/III \\
HD 119285  & = V851 Cen & 7.69 & 0.81 & 1.09 & 0.61 & 1.20 & K2/3~IV \\
HD 127535  & = V841 Cen & 8.55 & 0.82 & 1.08 & 0.61 & 1.18 & K2/3~IV \\
HD 136905  & = GX Lib   & 7.30 & 0.73 & 1.03 & 0.57 & 1.10 & K1~III + G5~IV \\
HD 139084  & = V343 Nor & 8.06 & 0.37 & 0.82 & 0.48 & 0.93 & K0~IV/V (PMS)  \\
HD 155555  & = V825 Ara & 6.87 & 0.38 & 0.82 & 0.49 & 0.93&G5~IV + K0~IV (PMS)\\
HD 174429  & = PZ Tel   & 8.44 & 0.31 & 0.78 & 0.45 & 0.87 & G9~IV/V (PMS) \\
HD 197481  & = AU Mic   & 8.73 & 1.12 & 1.45 & 0.94 & 2.06 & M1~V \\
\hline
\end{tabular}
\end{table}

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