Dynamical spectra of Na D lines. Top panels show the two Na D lines. Bottom panels show the same spectra after subtracting the average spectrum at orbital phases ±0.08 of the periastron passage. The red line represents the post-AGB component radial velocity curve according to the spectroscopic orbit of Oomen et al. (2018). The systemic radial velocity is marked with a dashed blue line, and the time of inferior conjunction is marked with a dashed black line. Orbital phase 0.0 corresponds to the periastron passage. The green line marks a time of enhanced absorption from the circumbinary disc component (hereafter region A).