levels. (See more detailed discussion below for Figure 17.)

Non-detections come in two basic varieties, those for which no significant evidence for O VI or C III emission lines is seen, and those observations for which significant stellar light contaminates the $\fuse$ data and makes detection of faint emission lines difficult to impossible. Figure 17 shows the C III and O VI regions of $\fuse$ data for SNR 0507-685, one of the non-detections without significant stellar contamination. This example shows both total and night-only data segments for comparison. No O VI lines are detected, and while some spectral structure appears at nearly the correct wavelength for LMC C III emission, the large change between the total and night-only data sets implies the observed feature is not intrinsic SNR emission. In the case of the SiC2A channel, this is an instrumental feature caused by the miscentering of low pulse-height photons from the strong airglow feature to the left. Still, the presence of such structure in the spectrum limits the ability to set a significant upper limit at C III unless a significant amount of orbital-night exposure time is available.

Figure 18 shows the same spectral regions for SNR 0519-697 (N120), one 