Top. Lyman-α line of 55 Cnc. Weighted co-addition of exposures flagged “in” (F_in; solid blue line) and “out” (F_out; solid red line) of the possible transit of 55 Cnc b’s extended atmosphere. The solid black lines show the stellar intrinsic emission profile before (thin) and after (thick) absorption by the interstellar medium, in terms of flux (left axis) or β ratio between radiation pressure and gravity (right axis). The absorbed profile is convoluted with the instrumental line spread function to yield the green profile, which is fit to the data. Dashed vertical lines delimit the region over which the flux is integrated to produce the light curve in Fig. 5. Bottom. Transmission spectrum showing the absorption depth δ = 1 − F_in/F_out during the possible transit of 55 Cnc b.