COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 1915

                                  Konkoly Observatory
                                  Budapest
                                  1981 January 29

                                  HU ISSN 0374-0676



    DISCOVERY OF 6.15 MINUTE OSCILLATIONS IN THE COOL
             MAGNETIC Ap STAR HD 24712



    In 1978 the discovery of light variability in HD 101065,
Przybylski's star, with the remarkably short period of 12.14
minutes was announced in these Bulletins (Kurtz 1978). An extensive 
analysis of the light variations and new infrared spectra
led Kurtz and Wegner (1979) to suggest that HD 101065 is simply
an extreme example of the cool magnetic Ap stars rather than a
unique object as has often been suggested. One immediate inference 
of this suggestion is that other cool magnetic Ap stars
may also pulsate with hitherto unsuspected short periods.
    On the night of 27/28 October 1980 observing from the Wise
Observatory at Mitzpe Ramon, Israel, oscillations with a period
of 6.15 minutes were discovered in the cool magnetic Ap star
HD 24712. Extensive observations of this star are now being made
from the South African Astronomical Observatory at Sutherland.
Figure 1 shows small sections of the B light curves of HD 24712
obtained in December 1980 on two nights separated by six days.
On the first night shown the peak to peak variation is nearly
0.01 mag and unequivocal. On the second night shown the variation 
is slightly less than 0.002 peak to peak, but can still be
seen and stands out clearly in an amplitude spectrum. The


                        [FIGURE 1] 

 Sections of the Johnson B light curves of HD 24712 obtained on
 J.D. 2444576 and 2444582. The light curves extend for hours
 on either side of the sections shown with similar appearance.
 Each point represents a 10-s integration. Light curves have
 also been obtained on all the nights in between those shown
 with a decreasing amplitude from night to night.


observations extend for hours on either side of the small pieces
shown in Figure 1. Observations have also been obtained on all
of the nights between the two illustrated with steadily 
decreasing amplitude from night to night.
    The 6.15 minute variability of HD 24712 is thus established
with certainty. It has been observed on a dozen nights from two
different sites and with two different telescope-photometer
combinations. It is also apparent that the variability is multi-
periodic and seems likely that the major beat period is near to,
or the same as, the rotation period of this star of 12.448 days.
     HD 24712 is perhaps the second coolest Ap star known next
to HD 101065. It is a spectrum variable and has a magnetic field
which varies from 300g to 1300g during the 12.448 day rotation
period (Preston 1972). We suggest here that there exists a class
of cool magnetic Ap stars with short period light variability and
that the discovery and study of these stars will make an important
contribution to our understanding of stellar pulsation, magnetism,
abundance anomalies and the relationships among them.
    Photometric searches for these objects must be done using
high speed photometry with stable equipment from a good site.
Weiss (1978), using conventional photometry with two comparison
stars, suspected variability in U in HD 24712 with a period around
3 hours. It is quite possible that 3 hours was the beat period
between his sampling frequency and the actual frequency of variation 
in this star. Because of the possibility of beat periods
on the order of days, observations on one night are not sufficient
to rule out variability. Several 1 hour runs on different nights
would be desirable.
    We are in the process of carrying out such a program. An
extensive discussion of HD 24712 as well as other newly discovered
variables of this type will be published in the near future.
    We acknowledge the Wise Observatory of Tel Aviv University
and the Smithsonian Research Foundation Grant SFC-0-3005 for the
use of their facilities at Mitzpe Ramon, Israel.

                            D.W. KURTZ *
                            Department of Astronomy
                            University of Cape Town
                            RONDEBOSCH. 7700.
                            SOUTH AFRICA

* Visiting astronomer, The Florence and George Wise Observatory, Tel Aviv
 University, Israel.


References:


Kurtz, D.W., 1979, I.B.V.S., 1436.
Kurtz, D.W., and Wegner, G., 1979, Ap. J., 232, 510. [BIBCODE 1979ApJ...232..510K ]
Preston, G.W., 1972, Ap. J., 175, 465. [BIBCODE 1972ApJ...175..465P ]
Weiss, W.W., 1978, Astr. Ap. Suppl., 35, 83. [BIBCODE 1979A&AS...35...83W ]