shocks slower than $\sim 160 \VEL$. We have no independent way of checking which of these possibilities may apply in this case. If the former suggestion is correct, it would imply a rather clumpy distribution of O VI, at least in some objects. If the latter situation is true, it may provide a reason why few (only one to date) C III-only objects are observed. Assuming we are systematically seeing the redshifted sides of the shells, objects with slower shocks (and hence lower bulk motions) will tend to be buried (or mostly buried) underneath the strongly saturated MC C III ISM absorption line. We note that the observed C III profile in the spectrum of SNR 0527-658 appears to be cut off sharply on the blue side. Apparently in this case, the relative motions are such that the C III line is partially seen in spite of the overlying absorption.

In general, however, a combination of effects tend to complicate the situation with C III. The uncertainty in position of the SiC channels with respect to LiF1, the lower effective area, and the propensity for significant spectral comtamination by solar-backscattered C III all work against C III detection at various 