S 308 is a circumstellar bubble blown by the WN4 star HD 50896 .
It is one of the only two single-star bubbles that show detectable diffuse X-ray emission .
We have obtained XMM-Newton EPIC observations of the northwest quadrant of S 308 .
The diffuse X-ray emission shows a limb-brightened morphology , with a clear gap extending from the outer edge of the diffuse X-ray emission to the outer rim of the nebular shell .
The X-ray spectrum of the diffuse emission is very soft , and is well fitted by an optically thin plasma model for a N-enriched plasma at temperatures of \sim 1.1 \times 10 ^ { 6 } K. A hotter gas component may exist but its temperature is not well constrained as it contributes less than 6 % of the observed X-ray flux .
The total X-ray luminosity of S 308 , extrapolated from the bright northwest quadrant , is \leq ( 1.2 \pm 0.5 ) \times 10 ^ { 34 } ergs s ^ { -1 } .
We have used the observed bubble dynamics and the physical parameters of the hot interior gas of S 308 in conjunction with the circumstellar bubble model of ( ) to demonstrate that the X-ray-emitting gas must be dominated by mixed-in nebular material .