This morning, as part of the welcoming ceremonies opening this conference, we were reminded that Indonesia consists of over 13,000 islands and that the coat of arms carries the phrase: “Unity in Diversity.” We might take the same theme for the study of Wolf-Rayet Stars. There has certainly been no shortage of diversity over the 120 years since their discovery and we all keep hoping for unity; perhaps IAU Symposium No. 143 will provide it! The conference was opened this morning with three strokes of the ceremonial gong to chase away all the bad spirits. So far that symbolic action has been most successful but perhaps more likely due to the kind hospitality of our hosts and the special warmth of Bali. We owe them all a vote of thanks!

To set the stage for the symposium, a brief overview is given of the main thrust of the about 1250 papers on Wolf-Rayet star research published in the past decade, broken down into inventory, basic parameters and galactic distribution, atmospheres, binaries, intrinsic variability, mass loss, enrichment and evolution.

Current knowledge of Wolf-Rayet continua is reviewed and observed continuum energy distributions are compared with model predictions. Good agreement is found. For WN stars a large range in intrinsic color is found for each subclass and the so far unexplained large range in the observed reddening-free continuum indices can be understood on the basis of atmospheric models. The strength of the continuum discontinuities determines the value of the index. Because of severe blending by the emission lines in WC spectra it can not be decided whether the intrinsic colors of WC stars of a given spectral type vary over a range of values or whether all stars of a given spectral type are similar. The reliability of model calculations for Wolf-Rayet stars is further tested by comparing observed and predicted correlations of helium line strengths. Satisfactory agreement is found for the Kiel/JILA models but not for the Bhatia-Underhill models. Methods used to determine the reddening due to interstellar extinction are reviewed and their accuracies estimated.

The evidence is reviewed that Population I Wolf-Rayet stars have solar abundances, that they are surrounded by remnant disks formed from their natal clouds, and that their rate of mass loss is moderate. These properties are consistent with Wolf-Rayet stars being young objects recently arrived on the main-sequence rather than the evolved, peeled-down remnants of massive stars.

Theoretical modeling of line and continuum formation in Wolf-Rayet (W-R) atmospheres is reviewed. We examine the basic premises on which the models are built, and critically compare theoretical models with observation. Discrepancies between theory and observation are analyzed to determine their implication for our understanding of W-R atmospheres. Line formation, continuum formation, and the ionization structure of W-R envelopes are examined in detail. Abundance determinations are briefly reviewed.

Valuable model-independent information about the chemical composition of WR stars can be obtained by using lines of different ions arising at transitions between highly excited energy levels. We determine the abundances of He, H, C, N, O using theoretical line intensities obtained by solving statistical equilibrium equations for level populations for different subtype WR envelope conditions.

The chemical composition of WR stars strongly differs from the mean cosmic composition. In the case of WN stars nuclear processed CNO products of H-burning are revealed in the envelopes and He-burning products in the WC envelopes.

The abundance of oxygen in WC stars differs from the predictions of recent evolutionary calculations which account for the new 12C(α,γ) 16O reaction rate.

Wolf-Rayet stars represent an important stage in the evolution of massive stars, but are only poorly understood so far. For a better knowledge one might determine their effective temperatures, luminosities and atmospheric compositions. But the emission-line dominated Wolf-Rayet spectra were not accessible to a quantitative analysis for a long time, because “standard” model calculations for static, plane-parallel stellar atmospheres are not adequate to that type of stars.

We present the first VLBI radio observations of two WR stars (WR140 and WR146) and of two early type stars (HD167971 and θ1 ORI A). The derived size and brightness temperature suggest that in all cases the radio emission originates from two distinct components: a steady weak emission due to the ionized wind of the primary and a smaller diameter brighter non-thermal component, probably associated to a small region within the ionized envelope of the star.

The observation of Helium lines in the near-infrared proved to be a powerfull tool to investigate the Wolf-Rayet stars and more particularly the WN.

We present empirical measurements of line strengths (Equivalent Widths-EW) of WCE stars (WC4-WC7). Our database presently includes approximately 50 emission lines stretching from the UV to the NIR. We have data for approximately 40 stars, but the spectral range is not complete for each star. At present, there are approximately 10-20 EW measurements for each line in the database. The lines used in the classification of the WC subtypes, along with a few others, show a dramatic dispersion in strength (as expected) within the WCE subtypes. Many other lines of carbon and oxygen ions indicate only a moderate range among the subtypes. We have found that there are several lines that show almost no dispersion in strength from WC4-WC7. These nearly constant lines can be used to give empirical estimates of the brightness ratio of the W-R to companion star in WCE + OB systems, and can also be used to estimate interstellar reddening. The similarity of line strengths among several C III and C IV emission features in WCE stars suggests that the ionization level in the wind is not fundamentally different among WC4-WC7 subtypes. Another parameter (e.g. C/He?) may have a substantial influence on those lines used for the WCE classification.

The observed He II Pickering decrement is modelled using the escape probability method developed by Castor & Van Blerkom (1970) to solve simultaneously the equations of radiative and statistical equilibrium for detailed model hydrogen and helium atoms. All important radiative and collisional processes are incorporated in this nLTE model. We confirm values of 0.0 ≤ H/He ≤ 0.5 for WNE stars and 0.0 ≤ H/He ≤ 3.0 for WNL stars with considerable spread in each subtype.