This paper concerns the teaching of mathematics for future engineers, focusing on the theme of trigonometry. We claim that the use of trigonometry in engineering courses requires different kinds of connections: connecting different domains, different concepts, frames and registers. We use here the concept of connectivity, developed in the frame of e-textbooks analysis, to analyse online courses for future engineers in France. We evidence that these courses propose some connections; but their connectivity is not developed enough to meet the requirements of engineering courses.