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Published online by Cambridge University Press: 19 July 2016
The IUE-ULDA database (Version 3.0, complete until the end of 1988) contains about 3500 spectra of more than 500 AGN of different classes (see Courvoisier and Paltani, IUE-ULDA Access Guide No. 4 A & B, ESA SP 1153 A & B 1992). We selected 67 objects for which the variability properties could be investigated. For each object we estimated the amplitude of the variability by means of the standard deviation σ(λ) of the flux at different frequencies divided by the mean flux.