The aim of this paper is to argue that some day in the not-too-distant future one of the well-known X-ray pulsars, say, Cen X-3 (although better examples would be probably GX 301-2 or GX 1+4), will disappear from the X-ray sky. It will reappear again after some, say, tens of years and its pulse period will be then longer than it is now.

Determination of the values and confidence intervals for physical parameters and the orbital inclination of binaries from polarimetry is critically discussed. A new method based on X2 best fitting and incorporating constraints imposed by the canonical model is developed and applied to polarimetric data for Cygnus X-1. Whereas the X2 best fitting procedure yields an inclination of |i| = 79° for Cygnus X-1, the level of noise, and the model dependency of the parameter determination give rise to a very broad confidence interval around this value. Our method also indicates the accuracy of the polarimetric measurements necessary for more precise parameter determinations.

The 16.6 day periodic radio flares in Circinus X-l have been observed at 6 cm wavelength from February 1978 to May 1979. Two strong flares occurred in February 1978 followed by a long period of quiescence until flaring recurred in January 1979. The onset of the flares was however 18 hours earlier in the 16.6 day cycle than previous flares. Similar behaviour has been reported for the X-ray emission and it is concluded that both the X-ray and radio emission are caused by mass transfer at peristron in an eccentric binary with a 16.6 day period. Each radio flare generally shows a multipeaked structure with the first maximum being the strongest. There is also a systematic trend for the subsequent peaks to become weaker with each cycle and this suggests that the compact object precesses in relation to an accretion disc.

Results of five nights of photoelectric UBV photometry of HZ Her and of simultaneous ultraviolet spectroscopy with IUE are presented.

We have scanned 75 high dispersion spectrograms of HDE 226868 = Cyg X-1 with the PDS microdensitometer of the David Dunlap Observatory in preparation for a detailed spectrophotometric study of the system. The plate material is identical to that described by Bolton (1972a,b,1975) and includes those plates. We have determined velocities for all of the spectrograms by fitting a parabola to the bottom of each line profile. The lines were divided into three groups, hydrogen lines, He I lines, and high excitation lines (He II, Si III and IV, N III), according to excitation, and separate mean velocities were determined for each group on each plate. We have computed the orbital elements of the system separately for each line group in order to look for differences caused by the effects of mass transfer or stellar wind.

Photoelectric and spectroscopic observations of optical counterparts of X-ray sources in the LMC are described.

The first results are presented of an extensive spectroscopic study of HD 77581 = Vela X-1, based on several averaged Coude spectra. The profile of Hβ is phase-dependent and consists of two absorption components superimposed on one another. The profile of He II 4686 A is of the P-Cygni type around phase 0.7, whereas the line is in absorption with variable strength between phase 0.9 and phase 0.5 (with respect to the X-ray eclipse).

Intermediate and narrow-band photoelectric observations obtained at Biruni and Villanova Observatories from 1977 to the present, when combined with previous data, strongly suggest that light increases of about 0.5 mag occur at 6 year intervals. Possible explanations are considered.

The X-ray pulsator X Persei (4U 0352+30) is an intriguing object at both X-ray and optical wavelengths. From the X-ray data a period of 13.9 minutes now appears well-established, but early suggestions of a 22.4 hour periodicity remain unconfirmed (White et al.,1976, Becker et al., 1979). The nature of the X-ray emission is unusual; the source is about 3 orders of magnitude weaker than other X-ray binary pulsars, and the spectrum contains a strong high-energy X-ray component (Mushotzky et al., 1977). Long known as an irregular variable, X Per has been classified as a Be star with an 09.5 (III-IV)e spectral type and showing broad hydrogen emission and absorption features (Brucato and Kristian 1972, Hutchings et al., 1975). A 581-day period is indicated by the radial velocity study of Hutchings et al., (1975). This has been variously interpreted as an orbital period (about a black hole) or an apsidal period, the models having in common accretion on to a neutron star as the source of the pulsed X-rays. In 1977 an increase in brightness of the star was observed by Kalv (1977) and Guinan et al., (1978), and by April 1979 we had accumulated sufficient evidence to suggest that X Per undergoes periodic (≃ 6-year) outbursts (Dorren et al., 1979a).

The bright BOe star X Persei has been observed for over 50 years as an emission-line object and as a variable star. Throughout this time, the star displayed typical behavior for such an object. The discovery that it lies very close to the position of a weak X-ray source renewed interest in the star, which is now under close scrutiny. The probable association of X Persei with 3U 0352+30 has been subject to several discussions (Hutchings, Crampton and Redman, 1975; Margon, Bowyer, and Penegor, 1976; Hutchings and Walker 1976; Persi, Viotti and Ferrari-Toniolo 1977, 1978).

In this paper we expand upon a proposed model for SS 433. Specific attention is given to the manner in which the light travel time within the system places constraints upon the geometrical parameters defining the system. Consideration is also given to the physical situation which exists in the high Doppler shift emitting region. Implications of recent observations of “a 13-day” period spectral and light variability upon the central source are briefly discussed.

We have collected all available times of minimum, over 1650, for 23 regular, 7 short-period, and 4 long-period eclipsing RS CVn binaries using the card catalogues at the Sternberg Astronomical Institute and the Krakow Astronomical Observatory, as well as unpublished data. We examined them critically, discarded those grossly in error, and assigned weights. For AD Cap, RV Lib, and ε UMi there are virtually no data. For the remaining 31 we determined quadratic ephemerides by weighted least squares. Values of d1nP/dt are given in the Table with their errors. Roughly 2/3 are variable at greater than the 2 σ level. Significant decreases outnumber significant increases by almost 2:1. We show that light curve asymmetry produced by the distortion wave probably accounts for the relatively small, rapid fluctuations in the O-C curve but not the long-term period changes.

We employ a simplified dynamo model to describe the long term photometric spectroscopic behavior of the RS CVn stars. The essential feature of the model is that the stars are in nearly synchronous rotation, with the differential rotation slower and the rotational velocity higher than for a single evolved 1 to 2 solar mass star. The spot groups are formed by eruption of enhanced toroidal fields, which have areas at the photosphere of several tenths of the surface area; the sizes of these regions are shear limited. Estimates of the lifetimes of active regions and of flare energetics are presented. The RS CVn phenomenon is then related to both the evolutionary status and the orbital parameters of the binary system.

RS CVn systems [1] are close binaries consisting of two late-type stars, one of which has evolved to the base of the giant branch. In most cases, the systems appear to be the product of normal stellar evolutionary processes [2,3]. These systems have attracted much recent attention because they exhibit greatly exaggerated stellar activity in the form of strong chromospheric emission [1]. There is evidence that a large fraction of the surface is covered by dark spots [4]. They also exhibit radio emission [5,6], and quiescent and flaring soft X-ray emission [7-12]. If these stars are undergoing normal evolution, why then are they so peculiar?

For several years, a systematic program to observe RS CVn and similar binaries has been undertaken with the 46-m telescope of the Algonquin Radio Observatory. A number of large radio outbursts at 2.8 cm wavelength has been found from HR 1099, AR Lac, SZ Psc, UX Ari, and HR 5110. In several cases, simultaneous, or nearly simultaneous, observations of these stars were made by cooperating observers at (other) radio, optical, UV, and X-ray wavelengths. It is now clearly established that the mechanism responsible for the cm-wavelength radio emission is nonthermal gyrosynchrotron radiation in a volume whose characteristic dimension is comparable with the binary star separation. More generally, a semi-quantitative model for the radio flare activity of these stars seems to be possible using greatly scaled-up analogues of solar magnetic activity.

The photoelectric observations of the close binary RS CVn carried out at Catania Observatory from 1963 to 1973 are presented. A new period of 9.48 years for the retrograde migration of the wave-like distortion on the light-curve has been determined. The wave amplitude is found to undergo cyclic changes with a period of about 4.74 years.

Possible correlation between the wave amplitude and the inclination of the rotational axis and the spectral type of the spotted component for several RS CVn and BY Dra binaries is investigated.

Spectrophotometry of the bright RS CVn binary HR 5110 is presented. The data consist of 42 red spectrograms obtained with the DDO 74″ telescope. All spectrograms were scanned using the PDS microdensitometer system at the DDO and rectified intensity tracings were produced.

Details of photometric observations during the totalities of the primary and secondary minima of SZ Psc are analysed in the light of the “star spot model” for RS CVn type binaries. Presence of scatter during the totality of primary minimum and its absence during the totality of secondary minimum of SZ Psc provide evidence towards one of the assumptions of the model,i.e. the spot activity confines to one of the hemispheres of the cooler star.

The three systems differ in period, scale and degree of contact: RW Com is a contact system, RT Lac is semi-detached and CG Cyg is a detached system. Yet they share the properties of a variable asymmetric light curve in the UBV, Ca II H and K emission, apparent infrared excesses, and period variability. If it may be argued that this ensemble of symptoms has similar origins in the different systems, then our intensive studies of these three binaries may have implications for the nature of all systems showing the RS CVn properties. We feel that the evidence points to the presence of circumstellar matter in the three binaries. We pose the questions: does this material create the photometric pecularities, is it incidental to them or does it arise from some other light-curve perturbing source?

Extensive pe- and spectroscopic observations of the short period eclipsing binary XY UMa within the last four years confirm the star spot and stellar activity model, which was presented in 1975.

Novae (N) and Dwarf Novae (DN) are considered to belong to the class of the Cataclysmic Variables (CVs). The Novalike stars and recently discovered Polars (the AM Herculis-type stars) seem to have the same nature as CVs and probably are members of this class. It is usually assumed that CVs are close binary systems, each system contains a degenerate dwarf and a nondegenerate second component, their period being usually less than 16h (e.g. see: Robinson 1976). A second star is believed to overfill its Roche lobe and its matter flows onto the degenerated component. Then around the degenerated dwarf there arises an accretion disk. This disk and the hot spot where the interaction between the disk and outflowing stream occurs are the main sources of the blue and ultraviolet continuum and strong H and He emission lines.