Bright X-ray emission has been detected in superbubbles in the Large Magellanic Cloud ( LMC ) , and it is suggested that supernova remnants ( SNRs ) near the inner shell walls are responsible for this X-ray emission .
To identify SNR shocks in superbubble interiors , we have obtained HST WFPC2 emission-line images of the X-ray-bright superbubbles DEM L 152 and DEM L 192 and the X-ray-dim superbubble DEM L 106 .
We use these images to examine the shell morphology and [ S ii ] /H \alpha ratio variations in detail .

Of these three superbubbles , DEM L 152 has the highest X-ray surface brightness , the most filamentary nebular morphology , the largest expansion velocity ( \sim 40 km s ^ { -1 } ) , and the highest [ S ii ] /H \alpha ratio ( 0.4–0.6 ) .
Its [ S ii ] /H \alpha ratio increases outwards and peaks in sharp filaments along the periphery .
DEM L 192 has a moderate X-ray surface brightness , a complex but not filamentary morphology , a moderate expansion velocity ( 35 km s ^ { -1 } ) , and a low [ S ii ] /H \alpha ratio ( \sim 0.15 ) .
DEM L 106 is not detected in X-rays .
Its shell structure is amorphous and has embedded dusty features ; its expansion velocity is < 10 km s ^ { -1 } .

None of the three superbubbles show morphological features in the shell interior that can be identified as directly associated with SNR shocks , indicating that the SNR shocks have not encountered very dense material .
We find that the [ S ii ] /H \alpha ratios of X-ray-bright superbubbles are strongly dependent on the UV radiation field of the encompassed OB associations .
Therefore , a tight correlation between [ S ii ] /H \alpha ratio and X-ray surface brightness in superbubbles should not exist .
We also find that the filamentary morphologies of superbubbles are associated with large expansion velocities and bright X-ray emission .