Radiative energy flux changes of Pleione in the far-UV through the Be-shell→Be transition

We present far UV observations of Pleione made with the IUE satellite in the time-interval 1979-1991 which show, for the first time in the wavelength range 1250-3000 A, the dramatic changes of the far UV radiative energy flux between the Be-shell and the Be phases of a Be star. Between 1979, when Pleione exhibited a strong shell spectrum, and 1991, when it showed a Be-type spectrum: (a) The observed far UV radiative energy flux increased by more than a factor two. (b) The absorption bump at 2200 A showed large changes, clearly indicating that it cannot be used confidently for measuring the interstellar component of extinction in Be stars