COMMISSION 27 OF THE I. A. U.
      INFORMATION BULLETIN ON VARIABLE STARS
                   NUMBER 415

                            Konkoly Observatory
                            Budapest
                            1970 January 21

HIGH-FREQUENCY STELLAR OSCILLATIONS. III. A BRIEF REPORT

     Our program, which is an extension of one begun at
the Princeton University Observatory by Lawrence, Ostriker
and Hesser (and Reported in ApJ,148, L161, 1967; 153, L151,
1968; and 155, 919, 1969), consists of acquiring long
(~7200 sec, continuous, digitized records of the apparent
luminosity of various stars in the lower left-hand portion
of the HR diagram. These records are searched for low-
amplitude, periodic variability using modern autocorrelation
and power spectrum techniques; various noise statistics are
also derived which help to distinguish objects possessing
intrinsic but non-coherent variability from either quiet
objects or objects exhibiting periodic behaviour superimposed 
upon an otherwise quiescent radiation field. During
a two-month run on Cerro Tololo in early 1969 we surveyed
a large number of central stars of planetary nebulae (CSPN),
white dwarfs, old novae, U-Geminorum stars, optical candidates 
for X-ray sources and pulsars, and peculiar blue
stars. We were able to take advantage of the experience
gained at Princeton and, in so doing, we feel our techniques
have been measureably improved over what we reported at the
scientific meeting of Commission 27 at Prague.
     A partial summary of our results to date would include:
1) Discovery of a new, blue, short-period variable star,
G44-32, whose proper motion, magnitude and colors led Eggen
and Greenstein (ApJ 141, 83, 1965) to include it in their
catalogue of white dwarfs (EG 72). The star is variable
with amplitudes of about 2 % and periods in the range from
10 to 27 minutes. Our results have been described in detail
elsewhere (ApJ 158, L171, 1969). Spectroscopy and further
photometry are needed to clarify its nature and relationship 
to other short period blue variable stars such as
HZ 29.Should other investigators be interested in acquiring
substantial blocks of photometric data on G44-32 during the
forthcoming season in a form suitable for time series
analysis (in particular, equi-spaced data points),we would
pleased to consult with them concerning the possibilities
of analysing their data with our computer programs,

2) A comparative study of the power spectra of white dwarfs
and CSPN in the period range from 4 to ~ 720 sec has been
made in which we have found that the CSPN are extremely
quiet with regards to both intrinsic flickering and
coherent, low amplitude variability. For many objects observed,
individual Fourier amplitudes in the relevant period
range were much less than 0.001 mag. Thus, in constrast to
tentative results previously reported and based upon data
taken under conditions of poor seeing,at Princeton, we find
that the power spectra of the CSPN more nearly resemble
those of the white dwarfs than they do the old novae: we
are, however, presently attempting to appropriately
characterize the mean continuous power spectra of the
various types of objects observed in order to learn if more
subtle differences exist which are difficult to isolate in
the high resolution spectra.

3) Observations were obtained of old novae, U-Geminorum
stars and optical candidate for X-ray sources and pulsars.
In particular, extensive data on WX Cen were obtained in the
period range of ~ 900 sec > period > 4 sec and some exploratory 
data were obtained which extend this range to
0,2 sec. We find that in the more common frequency range
WX Cen is only marginally noisier than comparison objects
and that it exhibits no coherent noise. Its general noise
level was significantly less on the nights of observation
than corresponding noise levels determined for Sco X-1 by
ourselves and other investigators. The dissimilarities between 
the optical power spectra of Sco X-1 and WX Cen suggests that 
the latter's identification with Cen X-2 (Eggen,
Freeman and Sandage, ApJ 154 L27, 1968) remains tentative.

4) For Sco X-1 the power spectrum was computed from a data
set 44 min long recorded at the 60-inch telescope with
0.1 sec time-resolution. This spectrum shows no periodic
activity with amplitude greater than 0.002 mag for 120 >
period > 0,2 sec. Combining this result with that of
Hiltner and Mook (ApJ 150, 851, 1967), we note that Sco X-1
is aperiodic for the range, 3800 < P > 0.2 sec, where P is
the period. We can interpret the continouous power spectrum
for 205 > P > 0.2 sec to consist of two parts: a flat component 
due to the photon statistics of observing, and a
decaying part with power spectrum proportional to f-2 for
P > 45 sec. For P < 45 sec, the decaying part of the
spectrum cannot be seen over the flat component. Data from
more active phases of Sco X-1 are needed to extend the
analysis to smaller periods.
5) An attempt to identify optically the pulsar,PSR 0833-45,
gave a null result (see Nature 223, 485, 1969),

                 BARRY M.LASKER and JAMES E.HESSER
               Cerro Tololo Interamerican Observatory
                       La Serena, Chile