COMMISSION 27 OF THE I. A. U.
                  INFORMATION BULLETIN ON VARIABLE STARS
                              Number 3507

                                                        Konkoly Observatory
                                                        Budapest
                                                        21 August 1990

                                                        HU ISSN 0374 - 0676


  THE DISCOVERY OF RAPID OSCILLATIONS IN THE Ap STAR HD161459.

The cool Ap star HD161459 was monitored photometrically for 7.36 hr on the
night 16/17 July 1990 (JD2448089), as part of an ongoing program to detect
rapid oscillations among the chemically peculiar A stars. The photometry,
which consists of a series of continuous 10-s integrations, was obtained with
the University of Cape Town photometer attached to the 1.0-m Elizabeth
telescope of the South African Astronomical Observatory (SAAO). All the
observations were obtained using a Johnson B filter and a 30-arcsec diaphragm
with occasional interruptions for sky measurements. Because we were searching
for oscillations with periods in the range of 4 to 15-minutes, we did not


                            [FIGURE 1]
                            [FIGURE 2]


employ a comparison star. However, experience has shown that on good nights it
is usually possible to distinguish between slow changes in sky transparency and
rapid oscillations in the star. The data were corrected for coincidence
counting losses, sky background and mean extinction, in that order. Finally we
took non-overlapping four-point averages of the data. We also removed some
long-period (P>0.92 hr, nu<26 day~-1) variations which we attribute to the
slow changes in sky transparency mentioned above. Since the periods of such
variations are well removed from the periods of interest, their removal does
not affect the analysis of these data. The resulting light curve is presented
in Fig. 1.
   Figure 2 is an amplitude spectrum of the data in Figure 1 out to the Nyquist
frequency of 12.5 mHz for 40-s integrations. This amplitude spectrum was
computed using Kurtz's (1985) faster implementation of Deeming's (1975)
Discrete Fourier Transform algorithm for unequally spaced data.  The prominent
peak is at nu=1.39mHz (P=12.0 min.) with a semi-amplitude of 0.84
millimagnitudes. The inset in Fig. 2 shows that we have a convincing detection
of the rapid oscillations even in the presence of the sky transparency
variations. These variations raise the level of the noise dramatically at
lower frequencies.
   A sinusoid with the frequency nu=1.39 mHz has been fitted to the data by
least squares in order to optimize its amplitude and phase. This sinusoid is
shown as the solid curve in Fig 1. The fitted curve is in fairly good
agreement with the observations.
   The discovery of rapid oscillations in HD161459 brings to 16 the total
number of rapidly oscillating Ap stars known (Kurtz 1990, Martinez et al.
1990). Further high-speed photometry of this object will be acquired in the
near future.


                                         PETER MARTINEZ
                                         G. KAUFFMANN
                                         Dept. of Astronomy
                                         University of Cape Town
                                         Rondebosch, 7700
                                         South Africa



References

Deeming, T.J., 1975. Astrophys. Space Sci., 36, 137. [BIBCODE 1975Ap&SS..36..137D ]
Kurtz, D.W., 1985. Mon. Not. R. astr. Soc., 213, 773. [BIBCODE 1985MNRAS.213..773K ]
Kurtz, D.W., 1990. Annual Reviews of Astronomy & Astrophysics, 28, 607. [BIBCODE 1990ARA&A..28..607K ]
Martinez, P., Kurtz, D.W., Kauffmann, G. & Jonson, A.C., 1990., 
   Inf. Bull. Var. Stars No. 3506.