Mixed interactive systems seek to smoothly merge physical and digital worlds. In this paper we focus on mixed objects that take part in the interaction. Based on our Mixed Interaction Model, we introduce a new characterization space of the physical and digital properties of a mixed object from an intrinsic viewpoint without taking into account the context of use of the object. The resulting enriched Mixed Interaction Model aims at balancing physical and digital properties in the design process of mixed objects. The model extends and generalizes previous studies on the design of mixed systems and covers existing approaches of mixed systems including tangible user interfaces, augmented reality and augmented virtuality. A mixed system called ORBIS that we developed is used to illustrate the discussion: we highlight how the model informs the design alternatives of ORBIS. Categories and Subject Descriptors H.5.2 [User Interfaces] Theory and methods, User-centered design. D.2.2 [Design Tool...