Abstract
We report the results of a survey of high-frequency SiO maser emission from a sample of 34 late-type supergiant and AGB stars. Four new sources were detected, including the first detection of 301-GHz emission from an S-type star. Variability of the maser emission with stellar phase is discussed in detail. We find that high-frequency SiO maser emission appears to be exceptionally weak or absent in Mira variables over an optical phase range of approximately 0.4 to 0.7. No significant correlation is found between maser photon luminosity and spectral type for non-supergiant types. Centroid velocities of spectra appear to be redder than the stellar velocity over a phase range that is wider than the band where emission is weak on average. However, the conclusion that only the blue features appearing in spectra after an optical phase of 0.7--0.8 correspond to new masing objects should be viewed with caution.
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