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\IBVShead{4949}{5 September 2000}

\IBVStitle{HD 77191: Another Variable Solar Twin}

\IBVSauth{Lebzelter, T.}

\IBVSinsto{Institut f\"ur Astronomie, A-1180 Vienna, Tuerkenschanzstr.~17, Austria, email: lebzelter@astro.univie.ac.at}

\SIMBADobjAlias{HD 77191}{HIP 44303}

\IBVSkey{photometry}

\IBVSabs{The detection of light variability of the G0V HD 77191 is reported.}

\begintext

The University of Vienna runs two automatic photometric telescopes
(APT) in the Southern Arizona desert named Wolfgang and Amadeus. A
description of the telescopes and the used photometers is given by
Strassmeier et al.\ (1997). Beside other programs the telescopes are
used for monitoring the light change of long period AGB variables
(Lebzelter 1999, Kerschbaum et al.\ 2000).  The discovery of a new
photometric variable G star reported here has been made in the course
of a short time monitoring of a group of AGB stars for determining
their current brightness. Typically one datapoint is obtained per
night. The new variable has been used as a comparison star within this
program.

HD\,77191 has been observed together with the program star RT Cnc
(M5III) and the second comparison star HD\,76678 (G5). Observations
have been done with Wolfgang using Johnson $B$ and $V$ filter. HD\,77191
has been identified as variable by comparison of the light change of RT
Cnc relative to both comparison stars.

\IBVSfig{6cm}{4949-f1.eps}{Light change of HD\,77191}

Figure~1 shows the light change of HD\,77191 in both filters.  Data
have been derived relative to the second comparison star HD\,76678
($V=8\fmm39$, $B-V=0\fmm83$ from Tycho Catalogue, Perryman et
al.\ 1998).  Each datapoint is the mean of ten measurements. The mean
scatter of these measurements is 0\fmm004 in $B$ and 0\fmm003 in $V$,
respectively, therefore clearly smaller than the amplitude of the
variations observed.  Figure~2 presents the results of a Fourier
analysis of the data using Period98 (Sperl 1998). From the strongest
peak (0.099988 c/d) we derive a period of $10.0\pm0.2$ days.  Figure~3
gives the visual brightness versus phase.

\IBVSfig{7cm}{4949-f2.eps}{Result of a Fourier analysis on the data.
The upper panel gives the spectral window, the lower panel the
amplitude spectrum}

The light curve is obviously asymmetric. Inspection of the mean light
curve in Figure~3 as well as of the total light change (Fig.~1) reveals
a bump on the descending branch of the light curve around phase 0.5.
The light change is similar in both filters with an amplitude of about
0.04 magnitudes. The asymmetry of the light curve can also be seen as a
second peak in the spectrum of the Fourier analysis (Fig.~2).

\IBVSfig{7cm}{4949-f3.eps}{Phase plot of the light variations of
HD\,77191 using a period of 10.0 days. Data points between 1.0 and 1.2
are repeated for clarity}

No investigation on this star has been reported in the literature. The
Hipparcos catalogue (Perryman et al.\ 1998) gives a spectral type of G0
which is in agreement with the $B-V$ value of 0\fmm67 derived both by
Hipparcos and by our APT measurements. The Hipparcos parallax for this
star of $15.58\pm1.15$ milliarcseconds gives an absolute $V$ brightness
$M_{V}= 4.83$ (without any corrections for reddening). It is
therefore probably a main sequence G0 star, similar to our Sun. The
Hipparcos measurements indicate that HD\,77191 is a single star.

We propose that the reason for the star's variability are star spots on
its surface. Several stars similar to the Sun have been found already
to show such variability (Lockwood et al.\ 1997, Strassmeier et
al.\ 1999).  Strassmeier \& Bopp (1992) presented the results of
different star spot geometries on the photometric light change.
Following their parameter study the light curve of HD\,77191 may be
explained by the assumption of two spots of different size at different
latitudes on the star's surface (compare their Fig.~10 and 11).

The variability in $B-V$ is very small (total amplitude of about
0\fmm01), but indicates that the star is reddest during its light
minimum around phase 0.6. This is in agreement with ``dark'' spots on
the stellar surface.

The measured period of the light change of 10~d is then the rotation
period of the star at the latitude of the spot(s). This period is quite
long for an active G0 dwarf (compare Fig.~1 in Hall 1991).  Our data do
not allow to derive any results on differential rotation of this star.

On the one hand, HD\,77191 is very similar to our Sun in several
aspects. On the other hand, there is an obvious difference in activity
between the Sun and HD\,77191. This makes this star an interesting
object for further spectroscopic and photometric investigations to
understand variability of main sequence stars in that part of the HRD.

\medskip
{\it Acknowledgements:} This research has been supported by the
Austrian Science Fund under project number S7301-AST and P14365-PHY.
The author wishes to thank Klaus Strassmeier for helpful comments on
the manuscript.

\references

Hall, D.S., 1991, in: The Sun and Cool Stars, eds.~I. Tuominen,
D. Moss, G. R\"udiger; Berlin, Springer, p.\ 353

Kerschbaum, F., Lebzelter, T., Lazaro, C., 2000, {\it A\&A}, submitted

Lebzelter, T., 1999, {\it A\&A}, {\bf 351}, 644

Lockwood, G.W., Skiff, B.A., Radick, R.R., 1997, {\it ApJ}, {\bf 485}, 789

Perryman, M.A.C., Brown, A.G.A, Lebreton, Y., et al., 1998, {\it A\&A}, {\bf 331}, 81

Sperl, M., 1998, {\it Comm.\ Astroseismology}, {\bf 111}, 1

Strassmeier, K.G., Bopp, B.W., 1992, {\it A\&A}, {\bf 259}, 183

Strassmeier, K.G., Boyd L.J., Epand, D.H., Granzer, Th., 1997, {\it PASP}, {\bf 109}, 697

Strassmeier, K.G., Serkowitsch, E., Granzer, Th., 1999, {\it A\&A Suppl.}, {\bf 140}, 29

\endreferences
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