To-scale sketch of the Vela X-1 system. HD 77581 is shown in blue and the orbit of the neutron star is represented by the black ellipse. Orbital phases are marked and labeled; phases covered in the present work are shown in red. We emphasize that the orbital phases are equidistant in time for the present orbital shape. We assume i = 73°, that is, lower limit for inclination according to Joss & Rappaport (1984), and the distances are thus lower limits. The background shows an indicative image of the variable large-scale structures that are formed through the interactions of the neutron star with the stellar wind of the mass donor. This image is an artistic impression based on simulations published in Manousakis (2011). The observer is towards minus infinity along the y-axis.