Most of the optical light from AM Her systems is cyclotron radiation. Simple models for the spectrum and polarization are successful for some systems, but in others the spectrum rolls over more gently at high frequencies and is polarized over a wider range of frequencies than the models predict. This paper considers the emission from both the sides and top of the accretion column and the effects of oscillations in the shock height. These features lead to a slower roll over at high frequencies than is found in the simple models, but it is still not as flat as in some of the AM Her systems.

We discuss the arguments in favour of the suggestion that polars (AM Her systems) and intermediate polars (IP’s) have magnetic fields of the same order of magnitude, and form one single class of objects. The period distribution of magnetic cataclysmic variables is well explained if they evolve the same way as non magnetic systems, IP’s becoming AM Her systems after crossing the gap. We discuss some consequences of the limited magnetic moment distribution (1033 ≤ μ ≤ 1034 G cm−3) in magnetic CV’s, in particular for the existence of accretion discs in those systems.

A detailed understanding of how the infalling matter in accretion columns is decelerated is essential for the calculation of the emitted radiation. On neutron stars, the deceleration takes place mainly by the interaction of the plasma with radiation, at least for the high-luminosity sources. We report on our two-dimensional calculations of the hydrodynamic flow in such accretion columns. The radiation transport is treated in the diffusion approximation, and we are looking for a stationary solution for the velocity field. The dependence of the results on physical parameters, especially on the accretion rate is discussed. Due to the non-linearity of the problem it turns out that only in certain parameter ranges stationary solutions seem to exist. For accretion rates higher than a critical value there are no stationary accretion flows. This leads us to the conclusion that a time-dependent picture for the accretion is unavoidable.

The discovery of magnetic field strengths in the range of 500 million Gauss in the objects Grw+70°8247 and PG1031+224 has given enormous impetus to investigations of magnetic white dwarf stars. We have determined intensities of Balmer transitions for B > 108 Gauss as a function of field and find strong oscillations of the transitions strengths, which are of importance to the quantitative analysis of the observed spectra and the interpretation of polarimetric measurements in these objects.

Absorption features seen in the bright phase spectra of E1405−451 are identified with Zeeman features of Hα originating from cool (T-104-105 °K) material surrounding the accretion shock. The presence of apparently unshocked material surrounding the cyclotron emission region is surprising and may indicate that accretion occurs onto a larger fraction of the stellar surface than had previously been thought possible.

Previous shock models for the AM Herculis-type magnetic variables have assumed homogeneous accretion columns, with constant infall densities out to a characteristic column radius. The resulting energy distributions are strongly peaked in the visible to ultraviolet and are in disagreement with observations. We report the preliminary results of calculations with more general radial functions of the infall density. Exponential and gaussian profiles yield continua similar to those of the standard model. More tapered, power-law profiles produce continua which are flatter and in better agreement with observations. Comparisons are made to data for AM Her and E2003+225.

We present high-time resolution spectroscopy of two AM Her sources E1405−451 and E1013−477. For E1405−451, the Balmer emission lines profiles can be divided into a narrow component and a broad one. The amplitudes of the radial velocity curves of these components are respectively 265±30 km/s and 390±50 km/s. The orientation of the column determined from polarimetry is not compatible with the broad component being formed in the lowest parts of the column. Photometric and spectroscopic results on E1013−477 do not confirm the previous reported 103 min. period. Rapid variability (<1.5h) as well as long term modulation (>3.3h) is present in these data.

The emission regions on the white-dwarf primaries of AM Her type systems are suggested to be extended and offset from the magnetic pole similar to the oval arc of emission formed by the Earth’s auroral zone. In ST LMi and EF Eri, the emitting areas of the bremsstrahlung, the soft X-ray, and the cyclotron sources are shown to display a hierarchy with a small bremsstrahlung core being surrounded by a larger cyclotron halo. Core and halo are characterized by large differences in specific accretion rate with that in the bremsstrahlung core corresponding to a sizable fraction (≳10%) of the Eddington rate.

3A0729+103 (= BG CMi) is an intermediate polar discovered through its X-ray emission (McHardy et al. 1981, 1984). The orbital period is 3.235 hours and the rotation period is 15.2 minutes, For ephemeris and references on the source we refer to McHardy et al. (1984). We report here on optical (4025 to 5090 A) and ultraviolet (1200 to 3200 A) spectroscopy obtained, respectively, on Dec 1, 1984 and April 21, 1985. Our data show clear modulation of spectral features with the orbital period.

We summarize results of our ongoing analysis of optical and X-ray data on the AM Herculis type binary H0139−68 (see also Beuermann et al., 1984; Schwope and Beuermann, 1985).

Our 5-color photometric observations of the intermediate polar H2215−086/FO Aqr which were collected between 1982 and the end of 1985 reveal that the 20.9 min rotational period increases with a spin-down rate of (1.0±0.2) 10−10 ss−1. This is in marked contrast to the spin-up behaviour of other well studied DQ Her systems.

The pulse profile of the optical modulation as a function of wavelength changes drastically across the Balmer jump suggesting the presence of two distinct components of pulsed optical emission which furthermore are shifted in phase with respect to the X-ray maxima observed with EXOSAT.

The UV, optical and IR flux distribution can adequately be accounted for by an accretion disc model with a mass-flow rate Ṁ = 1017 g s−1 and an inner boundary which is ≈10 times larger than the radius of the white dwarf. Our results support the magnetic accretion torque theory worked out by Ghosh and Lamb which can account for both spin-up and spin-down of the accreting star depending on the value of the fastness parameter ωs.

We derive a magnetic moment μ ≈ 1.0 1033 G cm3 which will allow H2215−086 to evolve into an AM Her system once it has crossed the 2–3 h period gap.

The preliminary results of EXOSAT and contemporaneous optical observations of E1405−451 (V834 Cen) in 1985 and 1986 are presented. In the latter of the two observations the soft X-ray light curve was observed to be quite different to that seen in all previous observations, but similar to the optical light curve and the new soft X-ray light curve of E2003+225. A phase shift of the broad soft X-ray eclipse was also observed. The hard X-ray and optical light curves have also undergone small changes.

White light simultaneous photometry and linear and circular polarimetry is presented for the AM Her variable ST LMi. Linear polarization evident throughout the faint phase is tentatively attributed to electron scattering from gas in the accretion column at heights greater than 5Rwd above the surface. The values of inclination and magnetic colatitude are revised to take into account the height and extent of the cyclotron emitting region which are also determined. A model of two closely connected emitting regions is proposed to account for the asymmetries observed in the light curve and polarization data for this and other AM Her systems. The cyclotron and hard x-ray emitting regions are found to be coincident, which eliminates the region above the shock as the source of polarized light. The shock height is found to be too low and the soft x-ray flux too large for the standard radiative shock model to apply.

Multiband observations of the AM Herculis-type binary CW1103+254 show significant circular polarization (~ 13%) in the J band. Recently, a model with high temperature (kT ~ 20 keV) and small dimensionless plasma parameter (Λ ~ 10 4) was suggested for the emitting region. However, it gives negligible polarization in the J band. In this paper, a method, in which the J band polarization and the peak frequency v* of the spectrum are taken into account, is used to determine T and Λ. For the viewing angle θ = 80° and the magnetic field B = 30MG, we find that kT = 5.0 keV and Λ = 106. The temperature of the emitting region is close to the value (kT = 8.7 keV) derived for the region emitting cyclotron lines in VV Puppis. If these radiations arise from the post-shock regions, then these temperatures are significantly lower than those predicted by standard accretion models or the shock structure is inhomogenous and more complex than previously assumed.

There is observational evidence of the presence of young neutron stars in old binary systems. A likely explanation is that those neutron stars were produced in the collapse of old CO white dwarfs. We show how mass accretion on initially solid white dwarfs can leave central solid cores when dynamical instability sets in and we study the different effects of the existence of such cores on the outcome of the competition between thermonuclear explosion and gravitational collapse.

We investigated the effects of rapid mass accretion on a fully convective main sequence star of 0.3 M⊙, taking into account the heating of its photosphere by the kinetic energy of the infalling matter. We found that the star develops highly inhomogeneous structure consisting of almost unperturbed initial convective core and a radiative envelope composed of the accreted matter. Both regions are separated by a very stable temperature inversion layer. Due to such structure accreting convective star may significantly increase its radius (contrary to earlier suggestions).

From the values of period changes for 6 close binary stars the mass transfer rate was calculated. Comparing these values Mt with the values of shell masses Msh, the expression

was derived. The analysis of this expression points out the initial character of the outflow of matter, and one may determine the time interval of the substitution of the shell matter. So one may conclude that for a certain mass transfer rate, a certain amount of matter accumulates in the nearby regions of the system.

Certain cataclysmic variables may evolve into low mass X-ray binaries if the white dwarfs can steadily accrete sufficient mass to exceed the Chandrasekhar limit. We present spectra of a recurrent nova and a low mass X-ray binary which are very similar to each other, and are also unusual for the strengths of the observed He II emission. We suggest that this similarity is not coincidental, but is evidence for an evolutionary link between the two classes of objects. A hydrogen depletion in the accreting gas is implied from the emission line fluxes, and may be an important parameter in determining whether accreted gas remains bound to the white dwarf, enabling eventual core collapse to occur.

During two observation periods in 1984 and 1985 we monitored the faint optical counterparts of seven X-ray binaries with a CCD camera attached to the Cassegrain-focus of the 2.2 m telescope at Calar Alto , Spain. The observation periods were 1984, September 25–27 , and 1985, August 8–15.

The transient sources 4U0042+32 and 4U0115+63 showed no active state.

The search for an optical candidate of 4U0142+62 did not reveal any object mv ≤ 24 within the EXOSAT error box.

Within the EXOSAT error box of EXO2030+37 the reddest object was found to be very faint in blue in contrast to the blue Palomar plate.

The analysis of the other three observed sources 4U1837+04 , 1H1929+509 , and 4U2129+47 is not yet finalized , but no unexpected bright or faint state of the sources has been found. 4U2129+47 is still in its present low state (mv = 18) , lasting at least since 1983.

We report results on three low-mass X-ray binaries (LMXB) in the LMC, obtained with EXOSAT, IUE and ground-based (ESO) telescopes: LMC X-2 which appears to be “Sco X-l”-like, LHG 83 and LHG 87. The latter are two weaker sources first detected in the course of the HEAO-B LMC survey. They have faint optical counterparts of which LHG 87 was only recently identified by us. In X-rays, they are characterized by ultrasoft X-ray spectra possibly characteristic of black-hole primaries. LHG 83 furthermore shows evidence for X-ray ionization of the surrounding interstellar medium, similar to the He III region around the black-hole candidate LMC X-l. X-ray binaries with masses of compact objects in excess of 3 M⊙ and ultrasoft X-ray spectra are comparatively frequent in the LMC. We suggest that subcritical accretion onto black holes takes place in LHG 83 and LHG 87.