Tracking two co-users' activity: The case of a joint writing situation mediated by a numerical artefact. Our research aims at observing a joint and mediated writing situation. In the activity we studied, two users were asked to co-write a procedural text with the help of a numerical artefact, composed of three types of resources: a video showing the content to be described, a chat and a text editor. During this activity, numerical interaction tracks were rendered visible. Tracks concern interactions between users and interactions between users and resources. We qualified these tracks and their properties, which concerned either the user or the numerical resource "sparking off" tracks, or the use of tracks made by users. Then, we revealed how the observation of the mobilisation of these tracks by users allowed us to capture the development of activity and the appropriation of the numerical artefact by users. Finally we discussed possible future works, and we presented some design trails for "tracing systems".