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\IBVShead{4348}{23 May 1996}
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\ft DISCOVERY OF 30-MIN OSCILLATIONS IN THE Ap~Sr(EuCr)
STAR HD\,75425
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Using the Str\"omgren photometry of Martinez (1993) as a
guide, we have been searching for rapidly oscillating Ap (roAp)
stars in the southern hemisphere. The roAp stars are cool,
magnetic, chemically peculiar A-type stars  (typically
classified as Ap SrCrEu) that pulsate with periods in the
range 6--16 minutes and Johnson $B$ semi-amplitudes $\le
0.008$ mag. There are currently 28 accepted members in the
class (Martinez 1993). This Bulletin announces the discovery
of 30-minute oscillations in the candidate roAp star
HD\,75425. Although this star is indisputably variable, it is
not clear that it is a roAp star, as we discuss below.

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HD\,75425 is classified as Ap Sr(EuCr) by Houk (1978), who
remarks ``weak case; undetected visual double,
P=288$^{\circ}$, D=0\farcs 4, mags 9.9, 10.0.''
Martinez (1993) measured the Str\"omgren indices to be
V=9.584, $b-y=0.112$, $m_1=0.247$, $c_1=0.805$ and
$\beta=2.864$. The calculated dereddened metallicity and
luminosity indices are $[\delta m_1]=-0.064$ and $[\delta
c_1]=-0.116$, which indicate strong metallicity and heavy
line blocking in the Str\"omgren $v$ band, characteristics
which we associate with roAp stars.

On the night of 18/19 March 1996 (JD~2450161) we observed
HD\,75425 for 4.92~hr using the Radcliffe People's Photometer
attached to the 1.0-m telescope of the South African
Astronomical Observatory in Sutherland. The data were
acquired as continuous 10-s integrations in Johnson $B$
light. A half-hour oscillation was evident at the telescope,
as confirmed by subsequent analysis of the light curve. The
data were corrected for coincidence counting losses, sky
background and extinction. Some low-frequency
($\nu\le347~\mu$Hz) sky transparency variations were then
removed and the data were binned to 40-s integrations.
Figure 1 shows a $2\frac{1}{2}$-hour portion of this light
curve and the discrete Fourier transform of the full light
curve. The tallest peak in the Fourier transform is at
560~$\mu$Hz (P\,=\,29.8~min) and has an amplitude of
2.10~mmag. These oscillations have been confirmed by
subsequent observations acquired using the Modular
Photometer attached to the SAAO's 0.5-m telescope.
Comparison of the available light curves for this star
indicates that the amplitude is modulated. This may be
caused by rotation and/or by beating of two or more
independent pulsation frequencies.

The interpretation of the oscillations in HD\,75425 is not
straightforward. A period of 30~min is rather long for a
roAp star, but also rather short for a $\delta$~Sct star. It
would be interesting to demonstrate the existence of
$\delta$~Sct-type oscillations in an Ap star, but it is not
clear that Houk's Ap Sr(EuCr) classification is correct. The
$m_1$ index suggests a high metallicity indicative of Ap
stars. Given the 0\farcs 4 separation and the
apparent brightness of this double the possibility that this
system comprises a chemically normal star and a magnetic,
chemically peculiar star cannot be excluded. Since the stars
comprising this double have such a small magnitude
difference, both stars are probably inside the instability
strip, making it difficult to establish which is the
variable.

I acknowledge a fruitful discussion with Don Kurtz and thank
him for his helpful comments.
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PETER MARTINEZ
Department of Astronomy
University of Cape Town
Rondebosch 7700
South Africa
email: peter@uctvms.uct.ac.za\\
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\references
Martinez, P., 1993, Ph.D. Dissertation, University of Cape
Town.

Houk, N., 1978, {\em Michigan Spectral Catalogue of
Two-Dimensional Spectral Types for the HD Stars} - Volume 2,
Department of Astronomy, University of Michigan, Ann Arbor,
Michigan.
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