2025ApJ...994..154L

We present the first systematic search for “changing-look” (“CL”) behavior in the broad He II λ4686 emission line in quasars, utilizing repeated spectroscopy from the Sloan Digital Sky Survey. The He II line, originating from high-ionization gas and powered by extreme-ultraviolet photons, serves as a sensitive tracer of changes in the ionizing continuum. After applying strict spectral selection criteria and visual inspection to a parent sample of over 9000 quasars with multiepoch spectra, we identify a sample of 34 He II “CL” quasars that show a significant appearance or disappearance of the broad He IIλ4686 line. Compared with previously known Hβ “CL” quasars, the He II “CL” sample exhibits similarly strong continuum variability and broad-line flux changes, yet shows a preference for higher Eddington ratios and lower host-galaxy contamination. These results highlight the value of the He II line in studying the central variable engines of active galactic nuclei and uncovering a more complete census of extreme quasar variability. A comparison with Hβ “CL” further underscores the profound selection biases inherent in “CL” studies, especially those associated with line strength, host-galaxy contamination, and spectral signal-to-noise ratio.