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\begin{document}
\IBVShead{4443}{28 February 1997}

\title{HD\,193084: A new variable star}

\begintext

The detection of variability in
HD\,193084 (V\,=\,7.5; CD$-$30\degr 17822, SAO\,189139) is reported. HD\,193084 was
used as one comparison star for the two $\lambda$ Bootis stars HD\,193256 and
HD\,193281 during our survey to detect pulsation within this group. Both
$\lambda$ Bootis stars and the second comparison star (HD\,194170) turned out
to be constant with an upper limit of 3 mmag in Str\"omgren $b$ (Paunzen et
al. 1996; Paunzen et al. 1997). Observations were performed at ESO during three
nights (see Table 1 in Paunzen et al. 1997). A detailed description of the 
observation and reduction procedure can also be found in Paunzen et al. (1997).

Variability in the brightness of HD\,193084 is clearly evident in all three nights. Figure 1
shows the differential light curves of HD\,193084 and HD\,193256 for 
the first night in Str\"omgren $v$. Furthermore, the data for HD\,193256 $-$
HD\,194170 are presented to show the good  photometric quality of the night and to establish the variability for HD\,193084. 

Using the data of all three nights, a time series analysis results in a 
period of about 80 minutes and an amplitude of 20 mmag in Str\"omgren 
$v$. The high
statistical significance ($\approx$\,15$\sigma$) compared to the mean noise
level proves the found period. Since Figure 1 shows that a
semi-regular (or multiperiodic) behaviour is evident, these results are just 
first numerical estimations. 

In order to determine the nature of  variability, a search for
informations in SIMBAD was performed. Houk (1978) classified this star as B8\,V with
a quality flag 1 (best). Unfortunately, no photometric measurements in one of
the common systems (Geneva, Johnson or Str\"omgren) were found making a 
calibration impossible. Following this spectral type, a possible $\delta$
Scuti pulsation can be excluded since the hot border of the instability
strip ends at A0. Looking for other sources of variability among B-type
stars, a possible membership in the $\beta$ Cephei group (stars hotter than
B4 with luminosity classes II to IV) and Be group (emission line stars with
shells, periods\,$\gg$\,hours and amplitudes\,$\gg$\,0.1 mag, e.g. Pleione,
$\gamma$ Cas, S Dor, etc.) is very unlikely. One may speculate that HD\,193084
is an unrecognized spectroscopic binary with a pulsating A-type component.
Also a false classification by Houk (1978) cannot be ruled out. 

In order to unambiguously establish the location of HD\,193084 
in the Hertzsprung--Russell--diagram and thus to
determine the nature of variability, photometric and spectroscopic 
observations are very much needed. The author, therefore, encourages further
observations as well as collaborations for solving the nature of HD\,193084.

{\it Acknowledgements}: This research was carried out within the working group
{\it Asterose\-i\-s\-mology-AMS} with funding from the Fonds zur
F\"orderung der wissenschaftlichen Forschung (project {\em S7303-AST}).
Use was made of the SIMBAD database, operated at CDS, Strasbourg, France.

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\normalsize
Figure 1. The differential light curves for HD\,193084 $-$ HD\,193256 and
HD\,193256 $-$ HD\,194170 for the first night in Str\"omgren $v$
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\beginauthors
\authorsrightcol
E. PAUNZEN
Institut f\"ur Astronomie der 
Universit\"at Wien
T\"urkenschanzstr. 17
A-1180 Wien
e-mail: paunzen@astro.ast.univie.ac.at
\endauthors

\references

Houk, N., 1978, {\it University of Michigan Catalogue of Two-Dimensional 
Spectral Types for the HD Stars},  Volume III, Astronomy Dept., Univ. 
Mich., Ann Arbor, Michigan

Paunzen, E., Weiss, W.W., Kuschnig, R., 1996, {\it IBVS}, No.\,4302

Paunzen, E., Kuschnig, R., Handler, G., Gelbmann, M., Weiss, W.W., 
1997, {\it A\&AS}, in press

\end{document}

