Our ROSAT observations of γ Velorum obtained over several binary cycles reveal substantial, and repeatable phase-dependent X-ray (0.1—2.5 keV) variations. The derived X-ray light-curve shows a 4 x increase in flux at phase ∼ 0.5, which is shown to arise in shocked gas formed in the collison of the two stellar winds, viewed through a cavity in the WC8 wind with an observed opening angle of ∼ 50°. This colliding-wind X-ray emission has Lx ≥ 1032 ergs−1 and kT ≥ 2 keV. Our 2-D hydrodynamical models of the system predict copious X-ray emission from the wind-collision region, at a level and spectrum similar to that observed. Harder (1—10 kev) X-ray observations with the ASCA satellite are being sought to provide improved constraints on the luminosity, spectrum and absorption of the wind-collision emission, and the complex theoretical models involved.

The shock between the colliding winds in binary systems containing two massive stars accelerates particles to relativistic energies. These energetic particles can produce observable non-thermal radiation from the radio to γ-rays. The important physical processes in such systems are very similar to those we have proposed for non-thermal emissions from single hot stars, which have shocks generated by instabilities in the radiatively driven stellar winds. This paper discusses the theory and observations of non-thermal radiation in the radio, X-ray, and γ-ray regions from both single stars and massive binaries. Similarities and differences between the two types of systems are outlined. We discuss two important physical effects that apparently have been neglected in previous theoretical work on colliding wind binaries.

It has frequently been suggested that Wolf-Rayet stars are present in symbiotic systems, in particular in symbiotic novae. We dismiss that suggestion as far as it concerns Wolf-Rayet of Population I. There may, however, be similarities to WR stars found as central stars of planetary nebulae. — When treating wind phenomena in our binary configurations, there are common properties which justify that for certain investigations Wolf-Rayet and symbiotic systems are considered together.

The significance of 3D effects in simulating colliding winds in WR binaries is demonstrated for the examples V444 Cyg, HD 193793 and γ Vel. The effects decrease with increasing separation of the stars. A comparison with symbiotic systems is drawn and a simulation of the symbiotic system EG And is presented.

The stellar winds flowing out of the components of WR+OB binaries can collide and shock waves are formed. Stellar wind collision, particle acceleration by the shocks and generation of X-ray, γ-ray, radio and IR emission in WR+OB binaries are discussed.

Ever since the earliest work, X-ray observations have been central in the chequered development of the study of interacting high-velocity winds in binaries. This review assesses how confidently the observations of Wolf-Rayet stars may be interpreted as the result of colliding winds, drawing attention to the wide range of X-ray luminosities even among the relatively bright binaries. Although the data do not generally conform to the simplest binary models, some suggestions are made to reconcile theory and observation.

The X-ray source 1E 1024.0-5732, serendipitously discovered with the Einstein Observatory, was previously interpreted as a rapidly spinning neutron star, accreting matter from its massive early-type companion, the emission-line star Th35-42. However, new ROSAT data do not support the presence of a neutron star in this source. A more likely scenario, also indicated by recent optical spectroscopy, involves X-ray emission from the colliding winds of a WR+O binary. Surprisingly, this star remained unnoticed during an extensive optical search for new Wolf-Rayet stars, carried out in this region of sky. Stimulated by the discovery of this first X-ray selected WR star, we have undertaken a search for similar objects in the data from the ROSAT All Sky Survey.

In this paper, a review of spectroscopic evidence for colliding wind effects in WR+O binaries is presented with a particular emphasis on variability at ultraviolet wavelengths. In the optical, evidence for colliding winds comes from extra emission formed in the region of shocked gas and superimposed on the strong emission lines formed in the WR wind. At ultraviolet wavelengths, we observe the consequences of the fact that between the two stars the winds are prevented from reaching their terminal velocity because of the wind-wind interaction.

Colliding stellar winds are an important part of early-type binaries. In this paper I discuss the phenomenon, concentrating mainly on the basic hydrodynamics of colliding winds, and the physics of X-ray emission. The following topics are covered:

1) Basic physics: The basic characteristics of the shock-produced thermal X-ray emission, and discuss general trends of X-ray emission from colliding wind binaries (CWBs).

2) Hydrodynamic simulations: Recent calculations have found that the interface in colliding winds is usually dynamically unstable, with three distinct instabilities.

3) Gamma Velorum: recent ROSAT observations give much insight into colliding winds. I discuss recent hydrodynamic calculations pertaining to these observations.

4) Radiation Hydrodynamics in CWBs: Recent calculations have included the effects of both radiation fields on the wind hydrodynamics in colliding wind systems.