The 15 candidate stars, which are marked by * or ! in Table 1, and their trajectories with respect to the two rings, plotted in 3D. The northern and southern rings are marked by green and red ellipses, respectively, in the upper right corner with the northern ring at the origin. The number next to each star corresponds to the identifiers in Table 1, and the candidate star BD +61 2408 is labeled accordingly. The vectors representing the stars’ trajectories are pointing back in time, showing the direction the stars came from. Note that the vector lengths are arbitrary and are only intended to show the direction of stellar motion. Taking proper-motion uncertainty into account, the small blue circle at the end of each vector represents the error in the direction that the star could have originated from. At the 2.7 kpc distance to NGC 7538, 1 pc corresponds to 1.′3 in R.A./decl.