Comparison of optical and X-ray characteristics. Top panel: radial velocities of the Be star in ζ Tau, measured from the Hα line using the double Gaussian method (filled triangles from Nazé et al. 2024 and empty triangles from the appendix of Nazé et al. 2022c) and phased with the ephemeris of Nazé et al. (2022c). Vertical lines show when X-ray observations were taken. Orbital phases 0 and 0.5 correspond to conjunctions with the Be star and its companion in front, respectively, while phases 0.25 and 0.75 correspond to quadratures. Bottom panels: comparison between the X-ray absorbing column and various diagnostics: orbital phase as well as EW, central absorption depth, and peak separation of the Hα line. The colors are the same as in the top panel.