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Variability of C III and Si III lines in the ultraviolet spectral region of the magnetic Bp star a Centauri

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 $λλ$ 1175.5 and 1300Å, respectively. Investigation of the variability of flux in the core of depression at $λ$ 1775.5 Å indicate that the fluxes do not vary within errors of measurements. On the other hand, the fluxes in the core of depression at $λ$ 1300 Å 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.