Non-Periodic Phenomena in Variable Stars
                                               IAU Colloquium, Budapest, 1968


                 THE STANDSTILLS OF LIGHT OF Z CAM STARS
                PHOTOELECTRIC AND SPECTROGRAPHIC OBSERVATIONS

                           MARIE-CLAIRE LORTET

                      Institut D'Astrophysique, Paris


  This paper is a summary of a longer paper to be published in the new 
"European Journal of Astronomy and Astrophysics"; a detailed paper will 
also appear in the Supplement Series of this Journal.
  There is curiously an almost complete lack of photoelectric or 
spectroscopic observations of Z Cam stars during their standstills of 
light. The study of standstills may be however quite important for the 
understanding of the physical nature of U Gem stars. In particular, it 
is not understood why certain U Gem stars undergo standstills and not 
the others, although the physical characteristics (masses, dimensions, 
separation) do not appear to be different.
  Photoelectric observations with B and V filters (and three observations 
with a U filter) were obtained for two standstills of light of Z Cam and 
for one of RX And. The results allow two important conclusions to be drawn:
  1) During a standstill, the rate of outflow of energy in the U, B, V 
region is the same as the mean rate of emission during normal periods 
(that is, during periods when outbursts occur without standstills), 
within 20 or 25 per cent.
  2) Rapid brightness fluctuations are present, probably of the same 
nature as those observed for ordinary U Gem stars, at all stages of 
activity (outside and during outbursts).
  A spectrum of Z Cam in the middle of a 480 days long standstill was 
obtained in December 1966. The phase of the orbital period extended from 
0.11 to 0.56 during the exposure time. Thus, according to the 
photoelectric light curve of Z Cam over the orbital period (Kraft and 
al. 1968; Smak, 1968), the observation was made entirely outside the 
part of the cycle when the light-curve shows a bright shoulder, so that 
the G-star was never seen through the thickest part of the ring around 
the other star.
  The spectrum has been briefly described previously (Lortet-Zuckermann 1967). 
It is fairly similar to the spectra obtained for Z Cam and other U Gem stars 
during the decline after an outburst.
  The profiles of the hydrogen lines exhibit central emission (about 20 A 
in total width) and broad shallow absorption wings extending to about 
+-80A on both sides of the central emission.
  No evidence is found for a strong increase in mass loss from the red 
star during the standstill, but small features would be lost, both due 
to the lack of time resolution and spectral resolution (about 2 A).
  The absorption lines may be explained by Stark broadening at the surface 
of the G star, while the emission lines arise probably in the ring.


                              REFERENCES

1) Kraft, R. P., Krzeminski, W., Mumford, G. S., 1968. Astr. J. 73, S21.
2) Lortet-Zuckermann, M. C., 1967. C. r. hebd. Seanc. Acad. Sci. Paris, 265, 826
3) Smak, J., 1968, Colloquium on Variable Stars, Budapest, September 1968. CoKon No. 65 46