  The U. C. Berkeley Infrared Spatial Interferometer has measured the
mid-infrared visibilities of the carbon star IRC+10216 and the red supergiant
VY CMa. The dust shells around these sources have been previously shown to be
time-variable, and these new data are used to probe the evolution of the dust
shells on a decade time-scale, complementing contemporaneous studies at other
wavelengths. Self-consistent, spherically-symmetric models at maximum and
minimum light both show the inner radius of the IRC+10216 dust shell to be much
larger (150 mas) than that expected from the dust condensation temperature,
implying that dust production has slowed or stopped in recent years.
Apparently, dust does not form every pulsational cycle (638 days), and these
mid-infrared results are consistent with recent near-IR imaging which indicates
little or no new dust production in the last three years (Tuthill et al 2000).
Spherically symmetric models failed to fit recent VY CMa data, implying that
emission from the inner dust shell is highly asymmetric and/or time-variable.
