Dust formation by Wolf-Rayet stars is a powerful diagnostic of exceptional activity in their winds. Observed behaviour ranges from stars which make dust continually (e.g., WR 104, WC9d+B0.5V) to those which make dust for short episodes at periodic intervals (e.g., WR 140, WC7pd+04-5). In both examples, dust formation is related to colliding WR and OB stellar winds in binary orbits. We have been using IR photometry primarily from the SAAO and ESO and spectroscopy with ISO-sws to investigate systems having intermediate properties (WR 98a, WC8-9vd+?) or more complex behaviour (WR 48a, WC8ed+?).

Wolf-Rayet galaxies are a subset of blue emission-line galaxies, whose spectra show the signature of large numbers of WR stars, with ages in the range 1-10 x 106 yr. Mid infra-red observations are well suited to the study of their hot, massive stars since this spectral region contains many fine-structure nebular lines which depend very sensitively on stellar content. The observations of ISO have opened-up this window, which we are exploiting through a Guest Observer program with the Short Wavelength Spectrometer (sws), that includes NGC 5253, a nearby (4 Mpc) WR galaxy. NGC 5253 contains several young super-star clusters, no older than a few million years and has been the focus of many recent studies which have shown that the extinction is very high and patchy in the UV and optical. ISO observations of NGC 5253 are particularly important since the interstellar extinction at mid-IR wavelengths is low and because of its remarkably hot and young stellar population.

In general, observationally derived wind compositions of WR stars are in reasonable agreement with predictions from stellar evolution models for massive stars. However, Barlow et al. (1988) identified a major discrepancy for neon in γ2 Vel (WC8+O) using ground-based observations. The advent of the ESA Infrared Space Observatory (ISO) has allowed the study of neon in many more WC stars, using mid-IR fine structure lines ([Ne II] 12.81 μm, [NeIII] 15.55μm and [NEV] 14.32 μm). Willis et al. (1998) used ISO-SWS observations of WR 146 (WC5+O) to derive a neon abundance that was within the range expected theoretically. Here we undertake a study of WR 90 (HD 156385), the only (apparently) single WC7 star in our Galaxy, using ISO-SWS spectroscopy. The only spectroscopic neon feature in the mid-IR of WR 90 is [Ne III] 15.55 μm, in addition to numerous C IV and He II transitions (see Figure 1).

We present ISO-SWS spectra of B[e] stars. We find a wide diversity of spectral characteristics of B[e] stars, suggesting different origins for the circumstellar matter. Most B[e] supergiants show hot dust with weak amorphous silicate emission. MWC 300 has a warm dust shell with strong crystalline silicate emission; its evolutionary status is unclear.

In this review we present the ISO imaging of nebulae around Luminous Blue Variables. Three LBVs have been imaged with ISO: HR Car, AG Car and the LBV candidate G79.29+0.46. The ISOCAM instrument did not resolve the nebula around HR Car. However some nebular emission lines are seen in the spectral energy distribution of this source. A proper deconvolution of the images may resolve the nebula. For AG Car and G79.29+0.46 the nebula is clearly resolved. The structure and intensity distribution of the nebular emission is dependent on the wavelength, indicating a separation between the ionised matter and the dust in the nebulae. Some model calculations are presented for the G79.29+0.46 nebula.

We are motivated to study the Ne/He abundance ratio at the surfaces of Wolf-Rayet stars by evolutionary models, which have predicted enhancements by more than an order of magnitude over cosmic levels during the WC stage (e.g., Maeder 1991). This should occur early in the WC phase, as core-hydrogen is exhausted and the CNO bi-cycle gives way to the following chain of reactions for the Galactic stars:

The latest evolutionary models, which include the effects of interior mixing due to stellar rotation, indicate that the enhancement may be somewhat more gradual, but that the Ne/He ratio is essentially unchanged in its peak value, coinciding with complete H-depletion (e.g., Meynet, these Proceedings).

We present spectra obtained with the Infared Space Observatory (ISO) of the dust shells surrounding several Luminous Blue Variables (LBVs), both in our galaxy and in the LMC. The 20-45 μm spectra of R71, AG Car and Wra 751 show prominent emission features from crystalline silicates. The composition of the crystalline silicates in LBV dust shells is compared to that found in other types of objects, such as (post)-AGB stars and red supergiants (RSG). Both Wra 751 and AG Car have a high ratio of pyroxenes to olivines. This suggests that the grains in both stars experienced very similar processing, but that this processing has lead to a higher abundance of pyroxenes compared to RSG. The dust composition of the three LBVs discussed here suggests dust formation in a cool outflow not unlike those of RSG.

The timing of PNe formation around low-mass WC stars is unsettled with respect to pulsations early in the post-AGB phase, or later thermal pulses (e.g., Tylenda & Gorny 1997). The chemistry of the dust in the nebulae can be used to trace the mass-loss history. Using ISO-SWS spectroscopy, the PNe BD+30°3639 [WC9] and He2–113 [WC11] have been identified by Waters et al. (1998) to exhibit emission from C-rich dust (PAHs) in the surrounding envelopes at λ 15 μm, while O-abundant silicate features are present at longer wavelengths. Figure 1 shows the PAH features, which include additional WCPNe observations to extend the range of stellar spectral subtypes.

The UV — IR properties of the continuous energy distribution of single WR stars in the Galaxy and LMC are reviewed. The observed, dereddened distributions may be approximated as a power law Fλ ∼ λ−α with a non-Rayleigh Jeans range of α directly associated with the emission of excess flux over the full wavelength range and across all spectral subtypes. The observational results are compared to published NLTE models, where we find an important discrepancy in the frequency distribution of α for Galactic WN stars. A dependency of emission line equivalent widths on the continuum flux at line center is discussed; the systematic offset of the absorption line (but otherwise presumed single) objects is examined, and the observed behavior is compared to models and explained with simple theory.