This review outlines the impact of observations across the spectrum (X-rays to radio) on our understanding of the basic physical, chemical, wind and mass loss properties of Wolf-Rayet stars. Optical spectropolarimetry indicates that ∼ 15% of the WR stars have anisotropic winds, whilst the majority have globally spherically symmetric outflows. All WR stars probably have winds structured (clumped) on the smaller scale as evidenced from: thermal radio spectral indices, optical-UV continuum/line/polarisation variablity and time-series spectroscopy, and the ubiquity of wind X-ray emission. ROSAT results indicate LX(WR) = 1031–33 ergs−1 with kT ≃ 0.3keV. WR mass loss rates may be lower than previously thought: mean, clumping-corrected rates from radio data yield M(WN) ≃ 3 × 10−5 M⊙ yr−1 and M(WC) ≃ 2 × 10−5 M⊙ yr−1. Analyses of UV-optical-IR spectra lead to good constraints on Teff and L/L⊙ and confirm the chemical separation of the WN and WC classes: WN stars show H-He-C-N abundances reflecting CNO-burning products, and WC(WO) stars show He-burning products. ISO data are confirming the H-deficiency of WC winds, and reveal a substantially enhanced Neon abundance in WC stars.

The VIIth catalogue of Galactic Population I Wolf-Rayet stars provides improved coordinates, spectral types and b, v, K photometry of known WR stars, and it adds 62 new WR stars to the previous WR catalogue. The present census of Galactic WR stars stands at 218, including 117 WN stars, 87 WC stars, 11 WN/WC stars and 3 WO stars.

A number of 134 WR stars is currently known in the Large Magellanic Cloud. A summary of the content and format of the new, Fourth, LMC Wolf-Rayet Catalogue is presented.

Observations of WR stars in binary systems are discussed, emphasizing constraints on our knowledge of the binary frequency of WR stars, and of WR stars as a distinctive class of objects. Radial velocity orbits of newly discovered binaries, e.g., WR 29, a short period WN7+OB binary in our Galaxy, and SMC/AB 7, a massive WN+O7 binary in the Small Magellanic Cloud, are presented. New spectroscopic observations of binary systems with previously known orbits are also reported, showing in the case of WR 21 evidence of change of the orbital elements as derived from different spectral lines. An elliptic orbit for CV Ser is also illustrated.

Observations and predictions of interactions in close and massive binary systems are summarized. Tidal interactions are analyzed using a model which predicts the response of the stellar surface to the external gravitational force of the companion. The possible role of tidal interactions in determining the peculiar behavior of systems like HD 5980 and η Carinae is discussed.

For binaries with well-known orbital parameters we describe methods to restrict some characteristics of the WR star in the binary, such as Te and RWR(τ ≃ 1). The presence of a hot, luminous companion helps to address the question of spherical symmetry of the WR wind and reveal the ionization stratification in the wind. Also, we discuss the restrictions which can be imposed on the wind velocity law(s).

From a new compilation of spectral types and photometry (version April '98 on the Web) performed by van der Hucht for the Galactic WR stars, we have estimated the distance using two different luminosity-spectral type calibrations. Both determinations lead to similar results. On the basis of newly determined distances we have performed a quantitative analysis of the spatial density distribution for the WN and WC spectral types as well as for the whole WR sample and compared them to the one depicted by the large star-forming regions delineated by the population of young open clusters (YOCs). WR stars show a clumpy distribution similar to the one shown by the YOCs and the WN and WC spectral classes present different galactic distributions as well. Otherwise, variations of the WC/WN quotient do not show a clear correlation with the galactic metallicity gradient, on the contrary this ratio seems to be constant on regions of about 1 kpc in size and shows significant variations (even by a factor of 8) between the closest star-forming complexes.

A continuum λ 4244 Å light-curve of the eclipsing binary V444 Cyg (WN5+O6) is solved. The results generally agree with the previous ones obtained by Antokhin et al. (1997), with the exception that the conclusion on the slowness of the acceleration in the extended atmosphere of the WR component must be ‘softened’.

We have used the Australia Telescope Compact Array to search for radio continuum emission at λ 3, 6, 13 and 20 cm from a distance-limited sample of 36 southern Wolf-Rayet stars within 3 kpc of the Sun. The observations were taken between 1994 and 1997. Radio continuum emission has been detected from 16 sources at 3 cm. Of these 11 sources were detected at 6 cm, seven sources were detected at 13 cm and 2 sources were detected at 20 cm (Chapman et al. 1999).

We obtain the stellar parameters of eleven OB stars in Cyg OB2 using model atmospheres. Each of the analysed stars shows an enhanced Helium abundance. We releate this is to the young agae of the OB association and/or the low rotational velocity of the stars.

The availability of 8–10m telescopes now permit the routine observation of faint, heavily reddened peculiar emission line objects, which may provide new information on stellar evolution. One such object is NaSt1 (WR 122, IRAS 18497+0056), discovered by Nassau & Stephenson (1963) who proposed a WR classification because of its strong emission line spectrum. Massey & Conti (1983) identified NaSt1 as a peculiar WN10 star. However, subsequent work has argued against a genuine WR identification, favouring either B[e], O[e] or Ofpe/WN9 spectral types from optical and IR spectroscopy (e.g., van der Hucht et al. 1997). NaSt1 has recently received renewed attention, principally because it is extremely bright in the IR (K = 6.5 mag). Indeed, NaSt1 is currently used as a late WN-type IR spectral standard, despite its uncertain nature.

There are a number of Wolf-Rayet stars which, based on their emission line profile variations, were proposed to be WR+cc systems (Firmani et al. 1980; Cherepashchuk & Aslanov 1984). However, recent intensive monitoring with different observational techniques, has given rise to alternative explanations for the variability in these WR stars including the presence of a disk, rotating jet-like structures (Vreux et al. 1992), and corotating interaction regions (St-Louis et al. 1995).

For eleven single galactic WR stars, the continuum energy distribution from the UV to the radio was obtained from literature data. Although a power law with α = 2.79 could describe all the distributions, there appear to be deviations that depend on stellar type.

We report radial velocity studies of photospheric absorption lines from spectral time series of the late O-type runaway supergiant HD 188209. Radial velocity variations with a quasi-period ∼ 2 days have been detected in high-resolution echelle spectra and most probably indicate that the supergiant is pulsating. Night-to-night variations in the position and strength of the central emission reversal of the Hα profile have been observed. The fundamental parameters of the star have been derived using state-of-the-art plane-parallel and unified non-LTE model atmospheres, these last including the mass-loss rate. The binary nature of this star is not suggested either from Hipparcos photometry or from radial-velocity curves.

An analysis of the behaviour of the He II 5411 line wings is presented. The spectral classification of the companion is discussed on the basis of a simple geometric model of the colliding stellar winds zone.

We have identified five new candidate LBVs in the NE half of M 31 and provide age limits for these objects through the color-magnitude diagrams of their potential parent OB associations. We note that three of the five candidates are not in OB associations nor are they in prominent H II regions.