Recent developments in the spectra of shell stars. I - The stable shell stars

Two of the three previously known stable shell stars, HD 193182 and HD 195325, are described. A third such star, HD 179343, is identified and a marked intensity change is observed in an otherwise stable shell, HD 192954. No changes in the shell velocities or spectra of the stable shell stars have been observed for up to half a century. The underlying stellar spectra are restricted to spectral types B6 to B9. The individual shell spectra range in intensity but are qualitatively similar. A very broad Ca II K shell line component is found in several stars.