  The angular diameters of six oxygen rich Mira-type long-period variables have
been measured at various near-infrared (NIR) wavelengths using the aperture
masking technique in an extensive observing program from 1997 Jan to 2004 Sep.
These data sets span many pulsation cycles of the observed objects and
represent the largest study of multi-wavelength, multi-epoch interferometric
angular diameter measurements on Mira stars to date. The calibrated visibility
data of o Cet, R Leo, R Cas, W Hya, chi Cyg and R Hya are fitted using a
uniform disk brightness distribution model to facilitate comparison between
epochs, wavelengths and with existing data and theoretical models. The
variation of angular diameter as a function of wavelength and time are studied,
and cyclic diameter variations are detected for all objects in our sample.
These variations are believed to stem from time-dependent changes of density
and temperature (and hence varying molecular opacities) in different layers of
these stars. The similarities and differences in behaviour between these
objects are analyzed and discussed in the context of existing theoretical
models. Furthermore, we present time-dependent 3.08 micron angular diameter
measurements, probing for the first time these zones of probable dust
formation, which show unforeseen sizes and are consistently out of phase with
other NIR layers shown in this study. The S-type Mira chi Cyg exhibits
significantly different behaviour compared to the M-type Miras in this study.
