TT Ari was detected by Strohmeier et al. (1957) and is classified as a novalike variable. It was hitherto unclear whether TT Ari is a special type of dwarf nova (Warner, 1976) or an old nova (Cowley et al., 1975). Our group obtained a total of four IUE spectra between 1979 and 1981 in the short and long wavelength region. The first spectrum was taken in July 1979, when the system had a visual brightness of 11.3 magnitudes. The following two IUE observations in November 1980 revealed TT Ari in the lowest optical state (V = 14m.3) observed so far. The fourth spectrum was obtained during the rise to maximum in January 1981, when the system had an apparent magnitude of V = 11m.8. From this behavior, Krautter et al. (1981) concluded that TT Ari is a dwarf nova with extremely extended standstills as they are typical for Z Cam stars.

A review was presented of the currently available observational data on this evolved system, namely, uvby photometry (Kilkenny et al. 1978; paper II) radial velocities (Kilkenny et al. 1981; paper III) and a non-LTE atmosphere analysis of the primary component (Kudritski et al. 1981). Kudritski et al. conclude that T(pr) = 40000 ± 2500°K and log g = 5.3 ± 0.2 and from the spectroscopic data and light curve analysis (papers II, III) derive masses and radii for the primary and secondary components of M(pr) ≃ 0.25 M⊙, M(sec) ≃ 0.04 M⊙, R(pr) ≃ 0.16R⊙, R(sec) ≃ 0.09R⊙ respectively. Thus the primary component is a normal sd0 star whilst the secondary component is a most enigmatic object, perhaps a normal composition degenerate dwarf but of too low a mass to be on the main sequence, perhaps an evolved degenerate object but then the radius is too large. The published evolutionary models for this system (Paczynski 1980; Conti et al 1981) do not adequately describe its current status.

The cataclysmic variable star EX Hydrae has been observed with the High Resolution Imager (HRI) and the Imaging Proportional Counter (IPC) onboard the Einstein Observatory. The X-ray position is coincident within 3 arcsec of the optical position as measured on Schmidt survey plates. During a 15 1/2 hour observation with IPC we have searched for a modulation of the X-ray flux. Strong evidence for a 67 min period (one of two known optical periods) has been found in the energy range 0.1-3.5 keV with the IPC. The time dependence of modulations is used to discuss a model and evolutionary status of this close binary system.

Photoelectric observations in U, B, V, were carried out to study the possible rapid (during a night) fluctuations of the close binary system V1357 Cyg, including the X-ray source Cyg X-1. Assuming the existence of irregular fast light fluctuations in the X-ray source Cyg X-1,the conclusion can be drawn that during these observations in 1979 the amplitude of the variations did not exceed 0.04 mag.

PU Vul was discovered by Honda (1979) and Kuwano (1979) due to its conspicuous increase of brightness in the years 1977-79. The analysis of the archive data by Liller and Liller (1979) revealed that the star was about 16.0m - 16.5m since the year 1898 until October 1977, with occasional brightening to 15m in 1926 and 1955. Since November 1977 it started to flare up by 5m until April 1979 when it reached a maximum of 9.0m. After the flat maximum lasting to June 1979, the brightness of the object started to diminish (Nakagiri and Yamashita, 1980). It reached 14.0m in the autumn of 1980, but then it flared up again, and was again of 9.0m in the summer of 1981 (Belyakina et al., 1981). The latter authors have found that the B-V index diminished in the 1980 minimum.

Recent observations of an apparent soft X-ray halo around the dwarf nova SU UHa have led to speculation that this may well be evidence of the object having undergone a classical nova-like outburst within historical times (Cordova and Mason 1980). By combining the relationship between quiescent X-ray luminosity and speed class for classical novae and the observed X-ray luminosity of SU UMa we derive a distance dependent apparent magnitude at outburst for the object. Distance estimates for SU UMa and absolute magnitude ranges for classical novae then determine the apparent magnitude of an outburst more exactly. From the angular size of the halo and the absolute magnitude - ejection velocity relationship for classical novae we derive the approximate date of any outburst. Comparison with historical records does not reveal any promising candidates. An alternative interpretation of the halo in terms of scattering of soft X-rays from dwarf nova outbursts by interstellar grains is suggested.

It is shown that the assumption of a black dwarf secondary in Z Cha leads to a number of contradictions with well established theoretical and observational facts. In particular the model predicts the radial velocity K1 to be much smaller than is observed, the radius of the accretion disk to be smaller than is physically possible and a white dwarf mass which is inconsistent with the white dwarf's radius derived from eclipse analysis. Using the same arguments as for Z Cha it is also possible to exclude a black dwarf secondary in the similar systems OY Car and HT Cas.