A kit of parts and a system for centering an external element with respect to an implantable medical device is provided that includes: •An implantable medical device (20) for being implanted under the skin of a patient and having an internal housing with a light source (22) positioned such as to emit a light beam coaxial to a secondary axis (Z2), •An external element (10) having —a number, N>；2, of photodetectors (12a-12d) forming a polygon of N edges, normal to a primary axis (Z1), and —an indicator (4) indicating how the external element is to be displaced over the the skin to position the external element with the primary axis (Z1), being coaxial with the secondary axis (Z2), as a function of the energy received by each of the N photodetectors. A corresponding method for aligning the external element and the implantable medical device are provided.
