  We have monitored the SiO (v = 1, J = 2 -> 1) maser polarization in 17
variable stars (Miras, OH-IR stars, and supergiants) to investigate the
long-term persistence of masers. The 8 epochs of observations span 2.5 years,
thereby sampling multiple cycles for these stars with typical periods of ~1
year. The average polarization was 23% with a typical dispersion of 7%,
although the variability differed substantially from star to star. In the
Stokes q and u spectra of individual stars, a few strong maser features tended
to dominate the polarization, with the maser features persisting for less than
one stellar cycle for some stars, and for multiple cycles for a few stars.
Because individual masers are not resolved in our beam-averaged total intensity
spectra, we correlated the polarization spectra between epochs to measure the
characteristic lifetimes of the features rather than attempting to trace the
evolution of separate line components. We found that individual maser feature
lifetimes ranged from a few months or less to >2 years. These data indicate
that for the sample of stars as a whole, the masers are not reset at a
particular stellar phase.
