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\IBVShead{5089}{25 May 2001}

\IBVStitle{Eight New Small Amplitude Variables}

\IBVSauth{Lebzelter, Th.; Posch, Th.}

\IBVSinsto{Institut f.\ Astronomie, T\"urkenschanzstr.\ 17, A-1180 Vienna, Austria, 
e-mail: lebzelter@astro.univie.ac.at}

\SIMBADobj{SAO 15812}
\SIMBADobj{SAO 18231}
\SIMBADobj{HD 291070}
\SIMBADobj{SAO 90186}
\SIMBADobj{HD 103847}
\SIMBADobj{HD 170270}
\SIMBADobj{SAO 154153}
\SIMBADobj{HD 105036}
\SIMBADobj{RW Vir}
\IBVSkey{photometry}
\IBVSabs{We report the discovery of small amplitude variability of six cool stars}
\IBVSabs{and discuss the nature of their light change. Two unsolved variables detected}
\IBVSabs{by Hipparcos are discussed, too. Six stars of the sample belong to the}
\IBVSabs{SRVs, whereas in case of the other two objects variability is most likely caused}
\IBVSabs{by a spotted surface.}

\begintext

Long term photometric monitoring of variable stars with Automatic
Telescopes (APT) likely leads to the detection and characterization of
a number of new variables used as comparison stars.  Detections of new
variable stars in the course of our monitoring programme of AGB
variables with the Vienna APTs on Washington Camp, Arizona, have been
reported by e.g.~Lebzelter (1999).  In the present paper we analyze
data on further eight small amplitude variables.

\begin{table}[h]
\caption{Summary of our observations and their results. The two stars
listed at the bottom have been observed in the $V$- and $I$-bands, the
other stars in $B$ and $V$}
\begin{center}
\begin{tabular}{lccclll}
\hline
\lower6pt\hbox{Star} & $V$-amplitude & spectral & period & variability & Program & Comparison\\[-2pt]
     &   [mag]   & type     &   [d]  & type & star & star\\
\hline
SAO 15812 & 0.08 & M2 & 20--50 & SR/L & SS Dra   & HD 108720\\
SAO 18231 & 0.05 & K5 & 15--20 & SR/L & SZ Dra   & HD 179238\\
HD 291070 & 0.10 & M2 & 40--50 & SR/L & V352 Ori & HD 41061\\
SAO 90186 & 0.08 & M0 & 40 & SR/L    & TW Peg   & HD 209420\\
HD 103847 & 0.05 & G5 & 13 & solar type  & GK Com   & HD 104785\\
HD 170270 & 0.04 & K5 & 10 & SR/L    & V585 Oph & HD 169930\\
\hline
SAO 154153 & 0.3 & M0 & 25 & ?       & AC Pup & HD 70738\\
HD 105036  & 0.1 & M0 & 20 & SR/L    & RW Vir & HD 105061\\
\hline
\end{tabular}
\end{center}
\end{table}

Photometric data have been obtained between March and December 2000.
From mid July to mid September the telescope was closed due to the bad
weather season. Typically, every second night a data point has been
obtained. The accuracy of the individual measurements is better than
0\fmm02. The first six stars listed in Table~1 have been observed in
Johnson $B$ and $V$ filters, while SAO 154153 and HD 105036 have been
monitored in $V$ and $I$.  The latter two have been observed continuously
since November 1996.

\IBVSfig{10cm}{5089-f1.eps}{Visual light curves of the stars in Table~1}

\vskip-11pt
None of the eight stars has left any significant echo in the
literature, except for HD 103847, which will be discussed below. All
objects of our sample have been measured by the Hipparcos/Tycho mission
(van Leeuwen et al.\ 1997). For three stars, HD 103847, SAO 154153 and
HD 105036, usable parallax measurements exist due to this mission.
According to this, HD 103847 and SAO 154153 are both main sequence
stars, while HD 105036 is a supergiant. Due to the failure to measure a
parallax for the other objects, we classified these stars as giants or
supergiants on the basis of their visual brightness values. Jorissen et
al.\ (1997) have noted that small amplitude variability is very common
among red giants of spectral type K and M\@. It is therefore not
surprising that we detected variability of these stars. The Hipparcos
mission found variability already in two stars of our sample, namely HD
103847 and HD 105036. Both stars have been classified as unsolved
variables. The light amplitudes given by the Hipparcos catalogue are in
reasonable agreement with our data. According to the Hipparcos
catalogue none of the program stars is a close binary.

All of the eight stars show semiregular to irregular light changes with
periods of a few ten days and amplitudes up to 0\fmm1. In Fig.~1 we
show the light curves for these stars except HD 103847 and SAO
154153 for which light curves will be presented below. As the periods
of the variations are very short compared to the total time span of
observations, we show only representative parts of the light change for
the sake of clarity.  The scale on the magnitude axis is the same for
all objects (0\fmm18) except HD 170270 and SAO 18231, for which it
has been enlarged.  The K and M type giants can therefore all be
classified as either semiregular or irregular variables. The nature of
the light change and the time span of our observations do not allow a
more detailed classification. Similarly, we can give only a range for
the typical time scale of the variability. The severe problems in
determining an accurate value of the period have been discussed already
in an earlier paper (Lebzelter 1999).  We note that there is a clear
difference in the time scale and amplitude of the variability between
the K and the M type giants of our sample, the latter having longer
periods and larger amplitudes.

\newpage

Two stars of our sample seem to be worth a further discussion. HD
103847 has the earliest spectral type of the stars in our sample.
According to Strassmeier et al.\ (2000), this star shows H and K Ca II
emission lines. Together with its spectral type, luminosity and period,
it follows that HD 103847 is likely to show the same type of
variability as the sun, but at a higher level of activity. According to
our measurements, the $B-V$ variation of this star is very small (i.e.,
essentially below 0\fmm015). Therefore, we only show the variation of
the $B$ magnitude of this star in Fig.~2.

For the time shown, the light curve can be fitted rather well by a
sinus curve with a period of 13~d and a $B$-amplitude of 0\fmm05. A
period about twice as long cannot be ruled out. Further observations of
this star are needed to determine the period accurately. Determination
of the period is also complicated due to the fact that on longer time
scales, the amplitude changes. The variability of the spots is
occurring on a time scale similar to the rotation period of the star.
This is in accordance with the results of Strassmeier (1990) and
Strassmeier et al.\ (1997), who already reported amplitude and period
variations of similar stars, presumably being due to rapid starspot
variability.

\IBVSfig{6cm}{5089-f2.eps}{The $B$-band variation of HD 103847}

The second interesting star is SAO 154153. According to its parallax
the star is a M type dwarf. The Tycho catalogue lists a $B-V$ value of
1.592. Our measurements give a $V-I$ of 2.6, suggesting a spectral type
later than M0. Photometry would therefore be in agreement with the star
being either a giant or a dwarf. In the latter case (which is favoured
by the parallax measurement), SAO 154153 might belong to the BY Dra
group, meaning that the light variability is due to rotational
modulation of surface structures. Its visual light curve and $V-I$
variation are shown in Fig.~3. A serious obstacle to classifying this
object as BY Dra variable is, however, that the $V-I$ variation of this
star, compared to its $V$ amplitude, is untypically large for a spotted
star. While an amplitude in $V-I$ of about 0.1 to 0.15 has been observed
in spotted stars, in such cases the visual amplitude is still several
times higher (e.g.\ Strassmeier \& Ol\'ah 1992).  Amado et al.\ (2000)
discussed models of different types of spots (solid spot,
umbra/penumbra model, umbra/facula spot model) and their effect on
light and colour curves. According to this study (see their Fig.~3) a
similar $V$ and $V-I$ amplitude may occur for umbra/facula spots. However,
we could not find observational evidence for such a combination in the
literature.

We conclude that our data and the information found in the literature
are not sufficient for a reliable classification of this star.  If the
parallax measurement would be wrong, the results would also be in
agreement with a classification as semiregular variable. However, in
the Tycho catalogue this star was marked with astrometric quality `3'
(T40) indicating rather high photometric accuracy. This star clearly
needs further investigation to reveal its nature.

\IBVSfig{9cm}{5089-f3.eps}{The visual light curve and $V-I$ variation of
SAO 154153}

Finally we note that HD 105036 has already been used as a comparison
star for RW~Vir before (Wisse \& Wisse 1971). In that investigation the
authors found a small scale variability in RW Vir with an amplitude of
0\fmm05. Based on our data we assume that their result was influenced
by the variability of HD 105036. From our data we did not find that
kind of variability in RW Vir. The light change of RW Vir itself will
be presented and discussed elsewhere.

{\it Acknowledgements.} We thank Th.\ Granzer, Potsdam, and the
referee for useful comments on our manuscript. This research has been
supported by the Austrian Science Fund under project number S7301-AST
and P14365-PHY.

\references

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\endreferences

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