We have undertaken quantitative analysis of four LMC and SMC O4-9.7 extreme supergiants using far-ultraviolet FUSE , ultraviolet IUE /HST and optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing (Hillier & Miller 1998) are used to analyse wind and photospheric spectral features simultaneously. Using Hα to constrain Ṁ , He i-ii photospheric lines reveal stellar temperatures which are systematically (5-7.5kK) and substantially (15-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions, since derived temperatures generally yield consistent fits across the entire λλ912-7000 Å observed spectral range. In particular, we are able to resolve the UV-optical temperature discrepancy identified for AzV 232 (O7 Iaf + ) in the SMC by Fullerton et al. (2000).The temperature and abundance sensitivity of far-UV, UV and optical lines is discussed. 'Of' classification criteria are directly linked to (strong) nitrogen enrichment (via N iii λ4097) and (weak) carbon depletion (via C iii λλ4647-51), providing evidence for mixing of unprocessed and CNO processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via O ii lines in the O9.7 supergiant for which it is also found to be somewhat depleted. Unfortunately, He/H is very difficult to determine in individual O supergiants, due to uncertainties in microturbulence and the atmospheric scale height. The effect of wind clumping is also investigated, for which P v λλ1118-28 potentially provides a useful diagnostic in O-star winds, unless phosphorus can be independently demonstrated to be underabundant relative to other heavy elements. Revised stellar properties affect existing calibrations of (i) Lyman continuum

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905 -1187 Å with high spectral resolution. The instrument consists of four coaligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al:LiF coatings for optimum reflectivity from approximately 1000 to 1187 Å and the other two use SiC coatings for optimized throughput between 905 and 1105 Å. The gratings are holographically ruled to largely correct for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way as well as active galactic nuclei and QSOs for absorption line studies of both Milky Way and extra-galactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I and the strong electronic transitions of H 2 and HD.

A B S T R A C TNeon abundances are derived in four Galactic WC stars ± g 2 Vel (WR 11, WC81O7.5III), HD 156385 (WR 90, WC7), HD 192103 (WR 135, WC8) and WR 146 (WC51O8) ± using mid-infrared fine-structure lines obtained with ISO-SWS. Stellar parameters for each star are derived using the non-local thermodynamic equilibrium model atmospheric code of Hillier & Miller, together with ultraviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of g 2 Vel, we adopt very recent results from De Marco et al., who followed an identical approach.ISO-SWS data sets reveal the [Ne iii] 15.5-mm line in each of our targets, while [Ne ii] 12.8 mm, [S iv] 10.5 mm and [S iii] 18.7 mm are observed solely in g 2 Vel. Using a method updated from Barlow et al. to account for clumped winds, we derive NeaHe 3±4 Â 10 23 by number, plus SaHe 6 Â 10 25 for g 2 Vel. Neon is highly enriched, such that Ne/S in g 2 Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of 2 lower than predictions of current evolutionary models of massive stars. An imprecise mass loss and distance were responsible for the much greater discrepancy in neon content identified by Barlow et al.Our sample of WC5±8 stars span a narrow range in T p (55±71 kKY with no trend towards higher temperature at earlier spectral type, supporting earlier results for a larger sample by Koesterke & Hamann. Stellar luminosities range from 100 000 to 500 000 L ( , while 10 25X1 < _ MaM ( yr 21 < 10 24X5 Y adopting clumped winds, in which volume filling factors are 10 per cent. In all cases, wind performance numbers are less than 10, significantly lower than recent estimates. Carbon abundances span 0X08 < CaHe < 0X25 by number, while oxygen abundances remain poorly constrained.

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