Orbital plane of 9 Sgr. The ellipse shows the orbit of the secondary star about the primary (shown as the large blue dot at the origin of the axes). The tick marks indicate the positions of the secondary at the times of our XMM-Newton observations. The dashed line corresponds to the contact surface between the stellar winds at periastron, computed according to the formalism of Cantó et al. (1996). The projection of the Earth in this plane is located at (0, −D/a sini), where D is the distance of 9 Sgr.