  The variability of twice ionized lines of carbon and silicon in the
ultraviolet spectral region of the magnetic Bp star a Centauri is investigated.
This study is based on the archival {\it International Ultraviolet Explorer}
data obtained through the large aperture and in the low-dispersion mode. A
comparison of the average {\iue} high-dispersion spectrum of a Cen with full
synthetic spectrum as well as those including only lines of one element showed
that six C III and six Si III lines are responsible for the depressions of the
flux at $\lambda\lambda$ 1175.5 and 1300\AA, respectively. Investigation of the
variability of flux in the core of depression at $\lambda$ 1775.5 \AA\ indicate
that the fluxes do not vary within errors of measurements. On the other hand,
the fluxes in the core of depression at $\lambda$ 1300 \AA\ varies
significantly with amplitude of $\sim$0.2 mag. Moreover, the variability of
this depression are in anti-phase with helium lines in the visual spectral
region.
