profile of O dwarf stars that have clearly developed blueward NACs (marked above each doublet component). (TLUSTY models were used as the photospheric input in these cases.) ](nacs_fig3.png)

To demonstrate this application to O dwarf stars we fitted the C[iv]{} $\lambda\lambda$1550 $IUE$ line profiles of HD 60848 (O8 V pe var), HD 41161 (O8 V) and CPD -592603 (O7 V((f))). In each of these cases a well defined, relatively broad, NAC close to $v_\infty$ in weak P Cygni profiles is evident in $IUE$ high-resolution spectra. The SEI model fits are shown in Fig. 3. Adopting from the present study that the blue NAC forms at a velocity where the stellar photosphere covering factor is $\sim$ 1, the $q($C$^{3+})$ values obtained over the NAC region ($\sim$ 0.9 to 0.97 $v_\infty$) are $\sim$ 5.5 $\times$ 10$^{-10}$ M$_\odot$ yr$^{-1}$ (HD 60848), $\sim$ 1.2 $\times$ 10$^{-9}$ M$_\odot$ yr$^{-1}$ (HD 41161), $\sim$ 6.1 $\times$ 10$^{-10}$ M$_\odot$ yr$^{-1}$ (CPD -592603). These values are a factor of $\sim$ 3 larger than $q($C$^{3+})$ calculated over most of the profile (i.e. $\sim$ 0.2 to 0.9 $v_\infty$). An understanding of clumping as a function of wind velocity is clearly requisite for the determination of reliable mass-loss rate estimates. The UV 