We present results from a spectro-interferometric study of the Miras o Cet, R Leo and W Hya obtained with the Keck Aperture Masking Experiment from 1998 Sep to 2002 Jul. The spectrally dispersed visibility data permit fitting with circularly symmetric brightness profiles such as a simple uniform disk. The stellar angular diameter obtained over up to ~ 450 spectral channels spaning the region 1.1-3.8 microns is presented. Use of a simple uniform disk brightness model facilitates comparison between epochs and with existing data and theoretical models. Strong size variations with wavelength were recorded for all stars, probing zones of H2O, CO, OH, and dust formation. Comparison with contemporaneous spectra extracted from our data show a strong anti-correlation between the observed angular diameter and flux. These variations consolidate the notion of a complex stellar atmosphere consisting of molecular shells with time-dependent densities and temperatures. Our findings are compared with existing data and pulsation models. The models were found to reproduce the functional form of the wavelength vs. angular diameter curve well, although some departures are noted in the 2.8-3.5 micron range.
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