The peculiar early-type emission line supergiant S18/SMC : an opticaland ultraviolet study.

The authors have observed the extreme early-type emission line star S18/SMC during 1978 to 1983 with both optical and ultraviolet low and moderate dispersion spectra. The star is a pronounced variable in He II, C IV, N IV, N V and Si IV. There appear to be no associated photometric variations from λλ1200 to 10,000 Å. A model is presented for the envelope of this star, and it is argued that the mass loss rate is comparable to or greater than any of the stars in the IUE survey by Shore and Sanduleak (of order 10^-5M_sun;yr^-1). The nitrogen abundance is found to be considerably enhanced above solar, and indication of substantial mixing of processed material from the interior. Comparisons are drawn to a number of other stars with similar emission characteristics.