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