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\IBVShead{4188}{4 May 1995}                      % Use it with dummy or empty arguments

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\centerline{\ft HD\,147491 IS VARIABLE, BUT IT IS}
\centerline{\ft  NOT A DELTA SCUTI STAR}
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Short-period variability of HD\,147491 ($\alpha$ (2000.0) = 16\fm 23\fmm 23\fs,
$\delta$ (2000.0) = $-$26\degr 22' 16'', a few arcminutes north of the
globular cluster M4) was claimed by Yao \& Tong (1989, here after YT89) on the
basis of 3 nights of photometry. Their observations implied a period of about
25 minutes. YT89 also gave arguments that this star has mid-A spectral type,
using an estimate of the interstellar reddening of field stars in the direction
of M4 and published UBV photometry. They concluded that HD\,147491 is a new $\delta$
Scuti variable. Even earlier, Clementini (1979) suggested that this star is
``likely to be variable'', because her BVRI photometry disagreed with previously
published results.

However, the data of YT89 were acquired at Beijing Observatory, located at
a latitude of +40\degr, so that YT89 could never observe the star at air
mass values lower than 2.5. Moreover, they neglected the spectral classification
of HD\,147491 from the Michigan Spectral Catalogue (Houk 1982), giving a spectral
type of G0\,V as well as the star's H$\beta$-index of 2.598 measured by Eggen
(1983). Such a H$\beta$ value is not typical of a $\delta$~Scuti star.
Finally, YT89 find their estimated $(U-B)_0$ to be ``too blue''. If they
dereddened the $(V-R_c)$ and $(V-I_c)$ colors of Clementini (1979) in the same
way like the UBV photometry, they would have found further inconsistencies. On
the other hand, assuming that HD\,147491 is a G dwarf suggests that it is a
foreground star relative to the objects adopted for the determination of
interstellar reddening, and hence it could have lower reddening.

All this casts considerable doubt on the claim of YT89 that HD\,147491 is a
$\delta$~Scuti star. Therefore, it was decided to re-observe it in order to shed
more light on its nature.

{\bf Observations and reductions:}
In March and April, 1995, HD\,147491 was observed with the Modular photometer
attached to the 50\,cm telescope at the South African Astronomical Observatory,
a dedicated instrument for high-precision photoelectric photometry. Keeping in
mind the short period claimed for the star, the choice of the observing
technique must be considered carefully.

In order to detect periods shorter than one hour, high-speed photometric
observations can give excellent results (e.~g.~Handler et al., 1995), but this
technique is not very useful in the presence of sky transparency variations
(see Breger \& Handler 1993). Therefore, it was decided to acquire differential
photometry with two close comparison stars: this allows to compensate for
transparency variations as well as to examine the data for possible variability
of the comparison stars (the comparison star adopted by YT89 was HD\,147592,
for which the Michigan catalogue gives a spectral type of A0\,V). Furthermore,
by using close comparison stars the telescope can very quickly be moved from
star to star.

For these purposes, HD\,147592 (C1) and HD\,147649 (C2, A5\,III) were selected.
Both objects are within less than 20 arcminutes distance from the program star
HD\,147491 (P). For each star, a 50 second integration in the V filter as well as
a 10 second integration for sky background were made. The stars were observed in
the order: C1--P--C2--C1--P--C2--...
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\centerline{2}
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\begin{center}
Figure 1: Differential time-series photometry of HD~147491 (filled
circles), HD\,147592 (crosses) and HD\,147649 (diamonds)
\end{center}
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\centerline{\epsffile{4188-f2.ps}}
\begin{center}
Figure 2: Amplitude spectra and spectral window of the data of Figure 1.
The panels (from top to bottom) show the spectral window of the data, the
amplitude spectrum of the program star data, as well as the amplitude spectra
of the comparison star data.
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\centerline{3}
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\centerline{Table 1. UBVR$_c$I$_c$ photometry of the program and comparison stars}
\begin{center}
\begin{tabular}{cccccc}
\hline
Star & $V$ & $B-V$ & $U-B$ & $V-R_c$ & $V-I_c$\\
\hline
HD\,147491 & 9.587 & 0.613 & 0.066 & 0.365 & 0.740\\
HD\,147592 & 8.936 & 0.298 & 0.200 & 0.177 & 0.388\\
HD\,147649 & 9.648 & 0.442 & 0.271 & 0.277 & 0.575\\
\hline
\end{tabular}
\end{center}


Thus, consecutive measurements of HD\,147491 could be obtained about every 245
seconds (corresponding to a Nyquist frequency of 176 cycles per day), which is
more than sufficient to detect a conjectured periodicity of 25 minutes. During
the time-series photometric observations, the air mass never exceeded 1.07. As a
further check of data quality, the E-region standards E690 (A2\,V) and E629
(G5\,V) were measured at the beginning of each run.

All data were corrected for dead-time losses as well as for sky background and
mean extinction. Mean zeropoints (of all nights of observation considered
together) of P, C1 and C2 were calculated relative to each other and subtracted
from the time-series data. The zero-point subtracted data are plotted in
Figure 1, where the variability of the program star is evident. It is also
clear, however, that the light modulation occurs on a time scale much longer
than 25 minutes. Moreover, Figure 1 shows that only very small transparency
variations occurred during our measurements.


It should also be mentioned that the standard deviation of the zeropoints
between the comparison stars and the E-region stars were comparable to the rms
error of a single measurement of the standard stars. The rms errors of a single
measurement of the data displayed in Figure 1 are 1.9 mmag for HD\,147592, 2.1 mmag
for HD\,147649, but 6.0 mmag for HD\,147491.

${\rm UBVR_cI_c}$ photometry was also acquired for these three stars (Table 1);
the transformation into the standard system was secured by measuring several
E-region standards. We conservatively estimate the rms errors with 0.020 mag in
$V$, 0.010 mag in $B-V$ and $V-R_c$ as well as 0.015 mag in $U-B$ and $V-I_c$.
Note that HD\,147491 is variable and therefore the results for this star must be
treated with caution.

{\bf Analysis and discussion:}
Since the photometric conditions were very stable during our observations,
amplitude spectra can be calculated for each star separately, i.e. we do not
consider differential data between the stars. These amplitude spectra are shown
in Figure 2. Inspection of Figure 2 again suggests that the time scale of the
variability of HD\,147491 is longer than 25 minutes; it is at least several hours.
However, the spectral window is far too complicated and the data are too sparse
to make a more definite statement about the real time scale. In order to examine
the possibility that the low-frequency variability could ``mask'' shorter-period
variations, we have also calculated an amplitude spectrum for the HD\,147491 data
after removing the nightly zeropoint variations; once again there is no hint for
a 25-minute periodicity.


What kind of variable can HD\,147491 be? Considering the $\rm UBVR_cI_c$
photometry in Table~1 as well as uvby$\beta$ data for the star (Handler 1995),
we find that it is unreddened. Hence, it is far too cool to be a $\delta$ Scuti
star.

Applying the calibration of Crawford (1975), we find an absolute magnitude of
$M_V=4.0$. Thus, our results are perfectly consistent with Houk's (1982)
spectral classification of HD\,147491 (G0\,V) as well as with colors measured by
Lee (1977), and Clementini (1979).\break
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\centerline{4}
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Due to the small amount of data we obtained, we cannot pinpoint the cause of the
light variability of HD\,147491. However, we wish to emphasize that the star
might be interesting for researchers studying variability of solar-type objects
caused by starspots. If HD\,147491 would be a related object, then its amplitude
is rather high (note that Eggen (1983) found the star to be fainter than
HD\,147649) and it would be one of the most active solar-type stars. The
photometric behavior of HD\,147491 clearly requires a more thorough study. A
spectrogram of the star with sufficient resolution to examine it for
chromospheric emission or for a possible secondary spectrum would also be useful.\\

{\bf{Summary and conclusions:}}
We showed that HD\,147491 is an early G-type main sequence star which exhibits
light modulation on a time scale of at least several hours. Contrary to
a previous claim, we demonstrated that it cannot be a $\delta$ Scuti star; it is
far too cool.

It is suggested that authors reporting low-amplitude variability of certain
objects critically check if the quality of their data is sufficient to assure
that their claims are not a result of spurious effects, e.~g.~caused by the
usage of a non-optimal observing technique. Quantitative statements about data
quality (such as the rms scatter of comparison star measurements) are
imperative.


{\it Acknowledgements:} This research was partially supported by the Austrian
Fonds zur F\"orderung der wissenschaftlichen Forschung under grant S-7304.

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% Call for macros \authors - \authorsrightcol - \endauthors
\beginauthors
G. HANDLER
Institut f\"ur Astronomie
T\"urkenschanzstra\ss e 17
A-1180 Wien, Austria
E-mail: HANDLER@ASTRO.AST.UNIVIE.AC.AT
\authorsrightcol
\endauthors

% Call for macro \references
\references

Breger, M., Handler, G., 1993, {\it Baltic Astronomy}, {\bf 2}, 468 

Clementini, G., 1979, {\it PASP}, {\bf 99}, 637 

Crawford, D.~L., 1975, {\it AJ}, {\bf 80}, 955

Eggen, O.~J., 1983, {\it MNRAS}, {\bf 204}, 377

Handler, G., 1995, {\it The Observatory}, submitted

Handler, G., Breger, M., Sullivan, D. J., et al., 1995, {\it A\&A}, submitted

Houk, N., 1982, Michigan Spectral Catalogue, vol.~3, Department of Astronomy,
University of Michigan, Ann Arbor

Lee, S.-W., 1977, {\it A\&AS}, {\bf 27}, 367

Yao, B.-A., Tong, J.-H., 1989, {\it IBVS}, No.\,3334 (YT89)

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