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\begin{document}
\IBVShead{4316}{5 April 1996}

\begin{center}
{\ft ROTATION PERIODS FOR FOUR LOW-MASS STARS IN THE TAURUS--AURIGA
REGION WITH Ca\,II EMISSION}
\end{center}

\vskip 3mm

\begintext

   We present the results of a long-term photometric monitoring campaign for
four stars in the Taurus--Auriga region that were found to have strong Ca\,II\,H
and K emission by Herbig et al. (1986). The BVR observations were obtained
during four runs from August 22, 1992 to December 29, 1995, at the Mt.
Maidanak Observatory, Uzbekistan, using a 0.48m telescope equipped with a
pulse counting FEU-79 photomultiplier tube. The mean error of one observation
of a program star is typically $\pm$ 0\fm 01 in V, B$-$V, and V$-$R. The
interval of observations, number of observations in V and B$-$V, limits of the
light variations in V filter, mean magnitude and colours are listed in
Table~1. Maidanak BVR photometry Data Bank is at Tashkent Astronomical
Institute and available to anyone interested in.

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\bigskip
\begin{center}
Figure 1. Phase diagrams for light curves in the V filter for 4 stars.
\end{center}

%\vskip 1cm plus 0.5cm minus 0.5cm
\vskip 5mm

   We report first detections of periodic light variations for Anon~1, LkCa\,1,
LkCa\,2, and LkCa\,11. A periodogram analysis reveals that all the Ca\,II stars
have significant peaks in their power spectra. The most probable periods of
these stars and their epoch of observations are listed in Table~1. Phase
diagrams for light curve in the V filter for four stars are displayed in
Figure 1.

\vfil \eject

\centerline{2}
\begin{center}
Table 1
\end{center}
\begin{center}
\normalsize
\begin{tabular}{ccccccccccc}
\hline\\
Name & Obs. & n1 & $\rm V_{max}$ & $\rm V_{min}$ & $\rm {<}V{>}$ &
$\rm {<}B{-}V{>}$ & n2 & $\rm {<}V{-}R{>}$ & Period & Epoch of\\
\ & Interval &&&&&&&&(day) & observ.\\
%\ & 1900$+$ &&&&&&&&& 1900$+$\\[5pt]
\hline\\
\ Anon  1 & 1992$-$95 & 60 & 13.37 & 13.58 & 13.45 & 1.85 & 38 & 1.84& 6.493 &
1992$-$95\\
\ LkCa  1 & 1993$-$95 & 51 & 13.67 & 13.76 & 13.71 & 1.45 & 33 & 1.71& 2.497$+$
& 1993$-$94\\
\ LkCa  2 & 1992$-$95 & 68 & 12.22 & 12.37 & 12.29 & 1.39 & 44 & 1.34& 1.364$
\ast$ & 1995\\
\ LkCa 11 & 1992$-$95 & 62 & 13.10 & 13.31 & 13.19 & 1.52 & 44 & 1.57&  1.5396
 &  1992$-$95\\[3pt]
\hline
\end{tabular}
\end{center}
\large

\begin{itemize}
{\footnotesize
\item[n1 - ] number of observations in V and B$-$V;
\vspace{-2ex}
\item[n2 - ] number of observations in V$-$R;
\vspace{-2ex}
\item[ $+$ - ] two periods (0\fd 713 and 1\fd 661) can be present, which
produce fully equivalent folded light curves with P=2\fd 497;
\vspace{-3ex}
\item[$\ast$  - ] another period (3\fd 71) produces fully
equivalent folded light curve with P=1.364 day.}
\end{itemize}


\vskip 1mm
It should be noted that there are several other significant peaks in the power
spectra for LkCa~1 and LkCa\,2. The periods, which correspond to these peaks,
are listed in the notes to Table~1. The spacing of the observations in time (one day) causes ``false" periods. These periods can be calculated from the equation:
$\bigl| \frac {1}{P}-\frac {1}{P_f}\bigr| = 1.0027\: (day^{-1});$ where:
$P$ - is ``true" period, $P_f$ - are ``false" periods. One of the periods is
``true", the others are ``false" periods. Both ``true" and ``false" periods
produce fully equivalent folded light curves. In order to choose ``true" period
it is necessary to carry out further monitorings with observations more
frequent than once a night.


   The periodic variability of these objects can be interpreted as the
rotational modulation of the stellar flux by a group of dark surface spots.
These four Ca\,II stars, together with LkCa\,3 (7\fd 2), LkCa\,4 (3\fd 3745),
LkCa\,7 (5\fd 6638), LkCa\,14 (3\fd 35), LkCa\,15 (5\fd 85), LkCa\,16 (5\fd 6), and
LkCa\,19 (2\fd 236) monitored by Grankin (1992, 1993, 1994), Vrba et al. (1993),
and Bouvier et al. (1993, 1995), form a sample of eleven objects from
Herbig et al.'s list with known rotational periods. Thus, there is a high
detection rate of rotational periods amongst Ca\,II stars in the sample
considered. It should be noted that Anon~1, LkCa\,4, LkCa\,7, and LkCa\,11 keep
their rotational periods in intervals from 4 to 6 years (see also Grankin,
1994). The stability of rotational periods over several years indicates that
the active region in each Ca\,II star remains on a definite meridian over this
time scale. Properties of the active regions for these stars are similar to
those of RS~CVn stars.

\vskip 0.6cm

\beginauthors
\authorsrightcol
Konstantin N. GRANKIN
Astronomical Institute
Astronomical str. 33, Tashkent
700052 Uzbekistan
e-mail: grankin@silk.glas.apc.org
\endauthors
%\vfill \eject

%\centerline{3}
\vskip 1mm

\references

Bouvier, J. et al., 1993, {\it Astron. Astrophys.}, {\bf 272}, 176

Bouvier, J. et al., 1995, {\it Astron.  Astrophys.}, {\bf 299}, 89

Grankin, K.N., 1992, {\it IBVS}, No.\,3720

Grankin, K.N., 1993, {\it IBVS}, No.\,3823

Grankin, K.N., 1994, {\it IBVS}, No.\,4042

Herbig, G.H. et al., 1986, {\it Astron. J.}, {\bf 91}, 575

Vrba, F.J. et al., 1993, {\it Astron. J.}, {\bf 106}, 1608

\end{document}

