Integrating computer-based information into the real world of the user, is becoming a crucial challenge for the designers of interactive systems. The Augmented Reality (AR) paradigm illustrates this trend. Information is provided by the AR system to facilitate or to enrich the natural way the user interacts with the real environment. We focus on the outputs of such systems, so that additional information is smoothly integrated with the real environment of the user. We characterize the integration of the computer-provided entities with the real ones using two new properties: compatibility and continuity. After defining the two properties, we provide the elements and analytical method needed for assessing them. We also empirically study the two properties in order to highlight their impact on the interaction. The CASPER system, developed in our teams, is used to illustrate the discussion.