Shell structure in stellar water masers

Double-peaked emission features have been observed in water spectra from six long-period variables. These spectra closely parallel the double-peaked OH emission in the same stars. These observations strongly support the circumstellar shell model for maser emission rather than the asymmetric shock model. The velocity separation for water is somewhat less than for OH, and we argue that the observed water lines originate in a shell whose radial distance from the star is less than the accompanying OH emission