STIS spectral tracings of the central star showing the He i λ4714 emission and absorption in our direct line of sight from 1998−2014. Phase in the 5.5 yr cycle are indicated next to each tracing, see Sect. 2. He i displays a modulation with the orbital cycle regarding the velocities and emission and absorption strengths. For example, during each event the absorption component vanishes. A long-term trend is also evident: the He i absorption strength increased relative to the continuum since 1998. The figure also shows the decrease of the Fe ii λ4631 emission and the appearance and disappearance of the He ii λ4686 emission.
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