Opacity in the upper atmospheres of active stars

We present FUV and UV spectroscopic observations of AD Leonis, with the aim of investigating opacity effects in the transition regions of late-type stars. The $\ion{C}{iii}$ lines in FUSE spectra show significant opacity during both the quiescent and flaring states of AD Leonis, with up to 30% of the expected flux being lost during the latter. Other FUSE emission lines tested for opacity include those of $\ion{O}{vi}$, while $\ion{C}{iv}$, $\ion{Si}{iv}$ and $\ion{N}{v}$ transitions observed with sti