DSS2-R image of the region containing CMa R1 (north is up, east to the left). The ellipse shows the approximate extent of the shell, and the arrows indicate the past positions of the runaway stars going back a few million years (as labeled in the image), according to the derived proper motions of HD 57682 and HD 54662, and the bow-shock orientation of HD 53974. The dashed blue lines represent the 1σ uncertainty on the axis of the bow shock associated with HD 53974, as discussed in the caption of Fig. 4. The stars HD 54662, HD 53974, and HD 57682 would have been at a coinciding position, located close to the center of the CMa shell, ~6, ~2, and ~1 Myr ago, respectively.