Relative orbit of the O-star in WR 21a around the Wolf-Rayet primary star. The orbit is projected on the plane defined by the line of sight (vertical dashed line) and the line of nodes (horizontal dashed line). This was calculated according to the orbital solution of Tramper et al. (2016) with an assumed inclination of 58.̊8. Both axes are in units of the solar radius; the Earth is located to the bottom at y = −∞ on the ordinate axis. The orbital motion is counterclockwise. Open circles indicate the position of the O-star between phases 0 and 1 by step of 0.1. Periastron (P) occurs at phase φ = 0.0 and apastron (A) at φ = 0.5. The filled circles indicate the binary configuration at each of the XMM-Newton observations. The passages through the lines of nodes (horizontal dashed line) occur at φ = 0.0335 and φ = 0.928, whereas the conjunctions (intersections between the orbit and the vertical dashed line) occur at φ = 0.316 and φ = 0.9935.