COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     NUMBER 182


                                      Konkoly Observatory
                                      Budapest
                                      6 February 1967




          18-MIN. LIGHT-VARIATIONS OF HZ 29

      HZ 29, a peculiar hydrogen-deficient white dwarf (Greenstein
 and Matthews 1957), was observed photoelectrically by the writer on
 eight nights in 1962 using the Crossley reflector of the Lick 
 Observatory; at the writer's request the star was observed also on a
 single night by Dr. W. Krzeminski with the 42-inch reflector of the
 Lowell Observatory. The visual inspection of all the tracings led to
 the conclusion that on some occasions quasi-periodic variations were
 present with the amplitude of the order of a few hundreths of a
 magnitude and period of about 9 minutes (Whitford 1962). The entire
 photoelectric material has recently been re-analysed by means of the
 autocorrelation technique and the surprising result has been obtained
 that the light variations of HZ 29 have strictly periodic character,
 their period being equal to about 18 minutes (instead of 9 minutes
 suspected previously). The apparent lack of periodicity in some
 tracings was entirely due to the instrumental "noise" which heavily
 masked the star's intrinsic variation. The latter is seldom visible on
 the original tracings and become apparent only in the composite hight
 curves based on several individual cycles (see Fig. 1).

      A detailed account of the photoelectric observations of HZ 29
 and their analysis will be given in an extensive paper to appear in the  
 forthcoming issue of "Acta Astronomica" (Vol. 17, Number 3, 1967).
 Here I wish to present the more important points.

 

      1. The light curve of HZ 29 has a shape of a slightly distorted
 double sinusoid (see Fig. 1). The secondary minimum is shifted toward
 0.6P and slightly shallower than the primary. The amplitude amounts
 to about 0.05 mag. in the ultraviolet and to about 0.03 mag. in the
 blue; no yellow observations were available.
 
      2. No definite interpretations of these variations can be
 offered at present though it is tempting to advance a hypothesis of the
 binary nature of HZ 29. The double structure of the absorption lines
 of helium (Greenstein and Matthews 1957) could then be interpreted
 in terms of the large amplitude radial velocity variations averaged
 over time during the relatively long exposure. A direct spectroscopic
 test of this hypothesis will be rather difficult but seems entirely 
 possible.
     
      3. More photometric observations are urgently needed in
 order to determine a more accurate value of period and to study its
 stability.



      January 31, 1967


                                             J. SMAK
                                      Institute of Astronomy,
                                     Polish Academy of Sciences
                                           Warsaw, Poland





 References

 Greenstein, J. L. and Matthews, M. S. 1957, Ap.J., 126, 14.
 Whitford, A. E. 1962, A.J., 67, 640.


                       [FIGURE 1]

Composite ultraviolet light curve of HZ 29 based on 185 observations
made on February 4 UT. 1962. Phases were computed using P=0.0122 day