We have monitored different types of late-type stars in the 22 GHz maser line of water vapor with single-dish telescopes and the Very Large Array over several years. We find that the emission pattern in the circumstellar shells of Semi-Regular (SR) and Mira variables mapped by the VLA remain stable on the scale of several years while the maser profiles from the single-dish telescopes vary strongly. Thus the strong variability is mainly due to incoherent intensity fluctuations of the individual maser lines, which have lifetimes less than a year.
In contrast to this, the variability in OH/IR stars and M-Supergiants is more regular and is mainly a response to the long-period variations of the central star. H2O maser in many OH/IR stars are “extinguished” most of the time, as the excitation temperature is only high enough during the phases close to the maximum of the variability.
The strength of the profile variability is decreasing as a function of the radial distance of the maser shell from the star. This gives a natural explanation for the increasing regularity of the maser variations along the sequence SR-, Mira variables, OH/IR stars, M-Supergiants.