The spectrum of V348 Sagittarii

The unique irregular variable V348 Sgr was observed at various epochs. The faintest magnitude found was V = 18.4. It shows brightness changes of delta-V greater than 6.4, which can occur within a few days. The spectrum is similar at maximum and minimum light, except for the forbidden lines, which remain at approximately constant brightness. In addition to the previously known C II and He I emission lines, a unique Ne I emission-line spectrum was observed. Using a curve-of-growth analysis, the abundance of C was found to be dominant in the chromosphere, with enhanced N and Ne abundances. The relation of this object to the R CrB stars and the Wolf-Rayet central stars of planetary nebulae is discussed. A model is suggested in which the original outer H-rich envelope had been expelled, and a final He-shell flash in the envelope of the remaining core formed a C-rich extended envelope. Dust clouds between the chromosphere and the H-rich planetary nebula cause the strong visible-light variations.