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\IBVShead{4297}{14 February 1996}    % Use it with dummy or empty a

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\centerline{\ft PHOTOMETRIC VARIATIONS OF THE}
\centerline{\ft  MARGINAL Am STAR HD\,143232}
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HD\,143232 (SAO 207224, $m_{V}$ = 7.1) was chosen as comparison star for a
survey to detect variability in $\lambda$ Bootis stars (Paunzen et al., 1995)
because of its classification as Am star (Houk, 1982). The photometric 
observations were done with the ESO 50cm telescope at La Silla (observer:
E. Paunzen) during the nights of 25/26 and 27/28 July 1994. An integration time
of 15 seconds in Str\"omgren $v$ and $b$ was chosen. HD\,143181 (SAO 207215, 
$m_{V}$ = 7.3, B9V) was used as comparison star. 
In addition, spectroscopic observations (observer: B. Duffee) were performed
with the 24-inch Helen-Sawyer-Hogg telescope located at Las Campanas. 

We classify HD\,143232 as kA7hA5mF2 which corresponds according to
Jaschek \& Jaschek (1987) to a marginal Am star.
Figure 1 shows the spectrum with a resolution of 109 \AA/mm. We
performed the basic CCD reduction steps (correcting for bias and flat field)
and normalization with standard IRAF-routines.
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\centerline{Figure 1. Classification spectrum of HD\,143232}
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\centerline{2}
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\centerline{\epsffile{4297-f2.ps}}
\begin{center}
Figure 2. The lightcurves of HD\,143181 ($+$) and HD 143232 ($\bullet$) for both nights in Str\"omgren $b$
\end{center}

\vskip 1mm
The following photometric indices were found in the literature (Hauck \&
Mermilliod, 1990 and Rufener, 1988):

%\begin{table}[h]
\begin{center}
\begin{tabular}{|c|c|c|c|}
\hline
b $-$ y & $m_{1}$ & $c_{1}$ & $\beta$ \\ 
\hline
0.142 & 0.199 & 0.951 & 2.815 \\
\hline
\hline
B2 $-$ V1 & $m_{2}$ & d & g \\
\hline
0.033 & $-$0.476 & 1.306 & 0.104 \\
\hline
\end{tabular}
\end{center}
\vskip 2mm

The dereddening procedure and calibration in the Str\"omgren system uses 
the results of Moon \& Dworetsky (1985), comprises calibrations given by 
Crawford (1979) and the iteration procedure described by Hilditch et al.
(1983). The bolometric correction was calculated with the values given by
Balona (1994). It results in B.C. = 0.04, $M_{V}$ = 1.49,
$T_{eff}$ = 7800\,K, log\,$g$ = 3.82 and E(b$-$y) = 0.025.
In the Geneva photometric system we applied
the calibration of Hauck \& North (1993) and get $T_{eff}$ = 7700\,K. Both
results are comparable within the error bars. Stellingwerf (1979) derived
a PLC-relation for stars at the MS: 

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log\,$P$ = $-$0.29\,$M_{B}$ $-$ 3.23\,log\,$T_{eff}$ + 11.96
\end{center}
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\centerline{3}
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\begin{center}Figure 3. Amplitude spectrum and spectral window for the merged data of both nights in Str\"omgren $b$
\end{center}

\vskip 3mm
With the calibrated values for HD\,143232 we get a theoretical period
of $P_{th}$ = 130 minutes.
Figure 2 shows the lightcurves of HD\,143232 and HD\,143181 for both nights
in Str\"omgren $b$. We computed an amplitude spectrum with a standard
Fourier technique (Breger, 1990) resulting in a period of $P_{obs}$ = 115
minutes and a semi-amplitude of 9 mmag in Str\"omgren $b$ (Figure 3). \\
Variability in marginal Am stars has been found before (Kurtz, 1984).
The controversy of variability in Am stars is still not settled (Wolff, 1983).
The absence of pulsation in classical Am stars is usually attributed to
diffusion. The helium content in the HeII ioniziation zone, which is the
primary mechanism for the pulsation, may be reduced to the point that the star
becomes stable against pulsation. At the same time, other elements that are
strongly supported by radiation pressure may be concentrated in the outer
portion of the atmosphere, thus producing the abundance anomalies
characteristics of Am stars.
The tools of asteroseismology would be very powerful to understand
the interior and physical processes of Am stars.
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\centerline{4}
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\
{\it Acknowledgement:} This research was done within the working group
{\it Asteroseismology-AMS}. 
and was supported by the
Hochschuljubil\"aumsstiftung der Stadt Wien ($\lambda$ Bootis Sterne). 
We acknowledge receipt of telescope time at ESO and UTSO.

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\beginauthors
\authorsrightcol
E. PAUNZEN$^{\star}$
B. DUFFEE$^{\star\star}$
\endauthors

\vspace{8mm}

$^{\star}$Institut f\"ur Astronomie, T\"urkenschanzstra\ss e 17,
A-1180 Wien, Austria\\
e-mail: paunzen@astro.ast.univie.ac.at\\

$^{\star\star}$Las Campanas Observatory, Chile \\
e-mail: boyd@charlie.ctio.noao.edu\\

\references

Balona, L.A., 1994, {\it MNRAS}, {\bf 268}, 119

Breger, M., 1990, {\it Communications in Asteroseismology}, {\bf 20}, 1

Crawford, D.L., 1979, {\it AJ}, {\bf 84}, 1858

Hauck, B., Mermilliod, M., 1990, {\it A\&AS}, {\bf 86}, 107

Hauck, B., North, P., 1993, {\it A\&A}, {\bf 269}, 403

Hilditch, R.W., Hill, G., Barnes, J.V., 1983, {\it MNRAS}, {\bf 204}, 241

Houk, N., 1982, University of Michigan Catalogue of 
Two-Dimensional Spectral Types for the HD Stars, Volume III, Astronomy
Dept., Univ. Mich., Ann Arbor, Michigan

Jaschek, C., Jaschek, M., 1987, The classification of stars, Cambridge
University Press

Kurtz, D.W., 1984, {\it MNRAS}, {\bf 206}, 253

Moon, T.T., Dworetsky, M.M., 1985, {\it MNRAS}, {\bf 217}, 305

Paunzen, E., Heiter, U., Weiss, W.W., 1995, {\it IBVS}, No.\,4191

Rufener, F., 1988, Catalogue of stars measured in the Geneva Observatory
Photometric System, 4th ed., Geneva Observatory

Stellingwerf, R.F., 1979, {\it ApJ}, {\bf 227}, 935

Wolff, S.C., 1983, The A-type stars: Problems and perspectives, NASA SP-463

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