Properties and nature of Be stars. 26. Long-term and orbital changes of ζ Tauri

Context. One way to understand the still mysterious Be phenomenon is to study the time variations of particular Be stars with a long observational history. ζ Tau is one obvious candidate. Aims. Using our rich series of spectral and photometric observations and a critical compilation of available radial velocities, spectrophotometry of H α , and $U\!B{}V$ photometry, we characterize the pattern of time variations of ζ Tau over about a century. Our goal is to find the true timescales of its variability and confront them with the existing models related to various aspects of the Be phenomenon. Methods. Spectral reductions were carried out using the IRAF and SPEFO programs. The HEC22 program was used for both photometric reductions and transformations to $U\!B{}V$. Orbital solutions were derived with the latest publicly available version of the program FOTEL, period analyses employed both the PDM and Fourier techniques – programs HEC27 and PERIOD04. Results. We derived a new orbital ephemeris $T_{\rm RV max.}=$ HJD (2447025.6 ± 1.8) + (132$^{\rm d}\!\!.$987 ± 0$^{\rm d}\!\!.$050) $\times E$. The analysis of long-term spectral and light variations shows a clear correlation between the RV and $V/R$ changes, and a very complex behaviour of the light changes. The character of the orbital light and $V/R$ changes varies from season to season.