  We present ASCA SIS observations of the wind-blown bubble NGC6888. Owing to
the higher sensitivity of the SIS for higher energy photons compared to the
ROSAT PSPC, we are able to detect a T ~ 8x10^6 K plasma component in addition
to the T ~ 1.3x10^6 K component previously detected in PSPC observations. No
significant temperature variations are detected within NGC6888. Garcia-Segura &
Mac Low's (1995) analytical models of WR bubbles constrained by the observed
size, expansion velocity, and mass of the nebular shell under-predict the
stellar wind luminosity, and cannot reproduce simultaneously the observed X-ray
luminosity, spectrum, surface brightness profile, and SIS count rate of
NGC6888's bubble interior. The agreement between observations and expectations
from models can be improved if one or more of the following ad hoc assumptions
are made: (1) the stellar wind luminosity was weaker in the past, (2) the
bubble is at a special evolutionary stage and the nebular shell has recently
been decelerated to 1/2 of its previous expansion velocity, and (3) the heat
conduction between the hot interior and the cool nebular shell is suppressed.
Chandra and XMM-Newton observations with high spatial resolution and high
sensitivity are needed to determine accurately the physical conditions
NGC6888's interior hot gas for critical comparisons with bubble models.
