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Published online by Cambridge University Press: 25 September 2017
An extensive study of ultraviolet spectra of Ap stars observed with the S2/68 TD-1 experiment has shown a large depression at 1400 Å in the spectra of Si and SrCrEu stars which is not present in the spectra of normal stars. This feature is attributed to Si II autoionization lines. More such lines seem to appear at longer wavelength in the UV and in the visible. We discuss the interest of an astro-physical plasma to detect and study autoionization lines in absorption.