Section of the COS spectrum of the white dwarfs WD2257-073 (left) and WD1942+499 (right) spanning the C IV doublet at 1548 Å and 1551 Å (top) and the C III multiplet near 1175 Å (bottom). The data are overlaid with a synthetic spectrum calculated for an abundance that gives the best fit to the C IV doublet. For the two CIV lines that are observed at the photospheric velocity, i.e. the blueward line of WD2257-073 and the unique line of WD1942+499, photospheric models predict strong CIII multiplets that are not observed.