COMMISSION 27 OF THE I. A. U.
                INFORMATION BULLETIN ON VARIABLE STARS
                            Number 3553

                                                     Konkoly Observatory
                                                     Budapest
                                                     10 January 1991

                                                     HU ISSN 0374 - 0676


           DISCOVERY OF RAPID OSCILLATIONS IN HD 19918

The southern (delta=-82d) Ap SrEuCr (Houk & Cowley 1975) star HD19918 was
monitored photometrically for 2.2 hr on the night of 6/7 October 1990
(JD2448171) as part of the Cape Rapidly Oscillating Ap star survey. The scope
and goals of the survey as well as the first results are described in a
separate publication by Martinez, Kurtz & Kauffmann (1991). HD19918 is the
sixth rapidly oscillating Ap (roAp) star to emerge from the survey since its
beginning in May 1990.
   The photometry was obtained using the University of Cape Town photometer
attached to the 0.75-m telescope of the South African Astronomical Observatory
(SAAO) in Sutherland. The observations comprise continuous 10-s integrations
through a Johnson B filter and a 30 arcsec diaphragm with occasional
interruptions for sky measurements. Because we were searching for oscillations
on time-scales of 5-15 min we did not observe comparison stars and no attempt
was made to transform our observations to the standard system.
   The reduction procedure was as follows: The data were corrected for
coincidence-counting losses, sky background and extinction, in that order.
Because no comparison stars were observed the data contain variations on
time-scales of 0.5 hr which were introduced by sky transparency variations. On
good nights these sky transparency variations will be of sufficiently low
amplitude and be gradual enough not to interfere with our search for rapid
oscillations. Their effect is simply to introduce a few high amplitude (1-4
mmag) peaks at low frequencies (nu<0.6 mHz) in the Fourier transform of the
observations (See Fig. 1). The Fourier transforms presented is this Bulletin
were computed using Deeming's (1975) Discrete Fourier Transform algorithm with
the modification suggested by Kurtz (1985).
   In Figure 1(a) we present the amplitude spectrum of the observations of HD
19918 on JD2448171 out to 12.5 mHz. Normally we would remove the low frequency
sky transparency peaks, but we deliberately show them in Fig. 1 so that the
reader may judge the data. The peak marked nu_1 suggests the presence of rapid
oscillations at 1.5 mHz with an amplitude of 0.9 mmag. We thus observed this
star again on the nights JD2448172 and JD2448217 to confirm the presence of
rapid oscillations at nu_1 =1.5 mHz. The JD2448172 data do not show a peak at
1.5 mHz but these data were twice as noisy as the data acquired on night
JD2448171. We will not discuss the JD2448172 observations further. In Figure
1(b) we present the observations acquired on night JD2448217. The 1.50 mHz
peak appears with a very good signal-to-noise in these data. Note the
difference in the amplitude of nu_1 in the two panels of Fig. 1. Such amplitude
modulation is commonly observed in the roAp stars. It arises is the following
ways: (1) through the beating of two, or more, unresolved oscillation
frequencies, (2) though the oscillations being observed with changing aspect as
the star rotates or (3) through intrinsic variations in the amplitude of the
oscillation frequency. Only a detailed study of the oscillations will reveal
relative importance of these factors in HD19918.


                           [FIGURE 1]
                           [FIGURE 2]


   In Fig. 2 we present a light curve of the observations acquired on night
JD2448217. In order to reduce the point to point scatter we have binned the
data to 40-s integrations by taking nonoverlapping 4-point averages of the
data. We have also removed some low frequency sky transparency variations to
facilitate the reader's perception of the oscillations. The solid line is a
least squares fit to the data of a sinusoid with frequency nu_1 =1.50 m Hz and
an optimized amplitude of 1.61 mmag. The fit is reasonably good.
   As of this writing we have high-speed observations of HD 19918 on 8 other
nights, most of which show the 1.50 mHz peak at a good signal-to-noise. One
must bear in mind that nights of excellent photometric quality are needed to
study the rapid oscillations in HD 19918 because the lowest airmass attained by
this delta=-82d star at Sutherland is 1.54. Further observations and a detailed
frequency analysis of the oscillations of HD 19918 will be presented in a
future publication.


                             PETER MARTINEZ and D.W. KURTZ
                             Dept. of Astronomy; Univ. of Cape Town
                             Rondebosch, 7700 South Africa



References:

Houk, H & Cowley, A.P., 1975. Michigan Spectral Catalogue of Two-Dimensional
     Spectral Types for the HD Stars. Volume 1, Dept of Astronomy,
     University of Michigan.  [BIBCODE 1975mcts.book.....H ]
Deeming, T.J., 1975. Astrophys. Space Sc., 36, 137. [BIBCODE 1975Ap&SS..36..137D ]
Kurtz, D.W., 1985. Mon. Not. R. astr. Soc., 213, 773. [BIBCODE 1985MNRAS.213..773K ]
Martinez, P., Kurtz, D.W. & Kauffmann, G., 1991. Mon. Not. R. astr. Soc.,
     in press. [BIBCODE 1991MNRAS.250..666M ]