COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1209

                                                    Konkoly Observatory
                                                    Budapest
                                                    1976 November 25 


      THE DOUBLE-MODE CEPHEIDS GZ Car AND UZ Cen

Pel (1) in his photometric study of Southern Milky Way
cepheids suspected that the intrinsic scatter in the light
curves of five cepheids (Y Car, GZ Car, UZ Cen, AP Vel and
AX Vel) was caused by double-mode behaviour. Observations of
Y Car, AP Vel and AX Vel (references 2, 3, 4 respectively) have
been analysed for the component periodicities present. However,
in the case of GZ Car, and UZ Cen only the primary periods are
known. I decided to analyse Pel's observations to try to 
determine any secondary periods present.
   The procedure adopted was to plot the B and V light curves
assuming the values for the primary periods quoted by Pel. Mean
light curves were drawn and the residuals from these mean light
curves calculated. These residuals were analysed in the period
range P > 1 day using a least squares Fourier technique developed 
by Barning (5). For each star, Fourier analysis of both the
B and V residuals identified the same period as being the most
significant secondary period. Given the primary and secondary
periods the observations were decomposed into their component
waveforms using an iterative technique (6). Fig. 1 illustrates
the decomposition for GZ Car and UZ Cen using the B observations.
Periods, amplitudes (deltaV, deltaB) and Julian dates of maximum
light for each waveform are listed in the table. The two modes of
pulsation present are identified as fundamental radial mode (=0)
and first overtone radial mode (=1).

 Star  Mode   P     DeltaV  DeltaB  JD (Max. light)
             days    mag     mag      2440000+


GZ Car  0   4.15885  0.16    0.23       742.6
        1   2.933    0.07    0.10       742.4
UZ Cen  0   3.33438  0.30    0.41       746.1
        1   2.355    0.06    0.09       746.2

   The period ratios, P1/P0, for GZ Car and UZ Cen are 0.7052
and 0.7063 respectively, consistent with the period ratios 
obtained for other double-mode cepheids. The amplitudes of the
secondary periods are in both cases by far the lowest known for
double-mode cepheids. This supports Pel's suggestion that the
excess scatter in some of his cepheid light curves may be caused
by a low amplitude double-mode phenomenon. However, as the scatter 
for other stars is considerably less than the scatter exhibited 
in the light curves of GZ Car and UZ Cen, it would be 
exceedingly difficult to determine the values of any other 
secondary periods.
   I am very grateful to Dr. M.J. Smyth for the use of his
least squares Fourier analysis computer program.


                       R.S. STOBIE

                  Royal Observatory Edinburgh
                        Blackford Hill
                  Edinburgh EH9 3HJ, Scotland



References:

1. Pel, J.W. 1976, Astron. Astrophys. Suppl., 24, 413 [BIBCODE 1976A&AS...24..413P ]
2. Stobie, R.S. 1972, Mon. Not. R. astr. Soc., 157, 167 [BIBCODE 1972MNRAS.157..167S ]
3. Oosterhoff, P.Th. 1964, Bull. astr. Inst. Netherl., 17, 448 [BIBCODE 1964BAN....17..448O ]
4. Stobie, R.S. and Hawarden, T. 1972, Mon. Not. astr. Soc. 157, 157 [BIBCODE 1972MNRAS.157..157S ]
5. Barning, F.J.M. 1962., Bull. astr. Inst. Netherl., 17, 22 [BIBCODE 1963BAN....17...22B ]
6. Stobie, R.S. 1970, Observatory, 90, 20 [BIBCODE 1970Obs....90...20S ]


                      [FIGURE 1]

   Fig. 1. Component waveform for GZ Car and UZ Cen
        (a) original B observations plotted with primary period
        (b) residuals from mean B light curve plotted with
            secondary period
        (c) B observations corrected for mean residual light curve
            plotted with primary period