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We analyse the ultraviolet variability time scales in a sample of 15 Type 1Active Galactic Nuclei (AGN) observed by IUE. Using a structure functionanalysis, we demonstrate the existence in most objects of a maximum variabilitytime scale of the order of 0.02-1.00 year. We do not find any significantdependence of these maximum variability time scales on the wavelength, but weobserve a weak correlation with the average luminosity of the objects. We alsoobserve in several objects the existence of long-term variability, which seemsdecoupled from the short-term one. We interpret the existence of a maximumvariability time scale as a possible evidence that the light curves of Type 1AGN are the result of the superimposition of independent events. In theframework of the so-called discrete-event model, we study the event energy andevent rate as a function of the object properties. We confront our results topredictions from existing models based on discrete events. We show that modelsbased on a fixed event energy, like supernova explosions, can be ruled out. Intheir present form, models based on magnetic blobs are also unable to accountfor the observed relations. Stellar collision models, while not completelysatisfactory, cannot be excluded.Comment: 13 pages, 10 figures. Accepted for publication in A&

AGN variability time scales and the discrete-event model