Time variation of SWS spectra of oxygen-rich Mira variables

Two M-type Mira variables have been observed with SWS01 over an entire light variability cycle. One of the target stars, Z Cyg, shows strong variation in the circumstellar emission. The 10 mu m and 20 mu m 'silicate features' became stronger relative to the photospheric emission at maximum than at minimum and the relative intensity of the 10 mu m to the 20 mu m bands increased at maximum, indicating the increase of the temperature of circumstellar dust grains. We interpreted the variation in terms of dust nucleation and subsequent grain growth after minimum. The other target star, T Cep, shows relatively small, but complicated variation in its SWS spectrum. This star has atypical 10 mu m feature, the 13 mu m feature, and CO2 bands in the range 13-17 mu m. The variation of these features are presented.