Titre: Mixing a Covert and a Non-Covert User 

Abstract: In many applications, knowing the existence of communication can result in dire consequences (e.g. military defense). Covert communication refers to any communication setup where users wish to convey information, while ensuring low probability of detection by other users, adversaries or network monitoring nodes. In this work, we established the fundamental limits of a two-user single-receiver system, where communication from User 1 (but not from User 2) needs to be undetectable to an external warden. Our fundamental limits shows a tradeoff between the highest rates (or square-root rates) that are simultaneously achievable for the two users. Moreover, coded time-sharing for both users is fundamentally required on most channels, which distinguishes this setup from the more classical setups with either only covert users or only non-covert users. Interestingly, the presence of a non-covert user can be beneficial for improving the covert capacity of the other user.