Optical spectra reproduced from Kehrig et al. (2016, their Figs. 2 & 6, here top & middle, respectively) and Kehrig et al. (2015a, their Fig. 3, bottom). Integral field spectroscopy was obtained with the Potsdam Multi-Aperture Spectrophotometer (PMAS) on the 3.5 m telescope at the Calar Alto Observatory (Almeria, Spain). Top: Hβ map with a cross marking where the spectrum in the middle was extracted from. Middle: Spectrum showing a prominent WR-bump associated with stellar emission underneath the narrow, nebular He IIλ4686 Å line, discussed in Sect. 4.1. Bottom: Integrated spectrum of the north-western region of I Zw 18, demonstrating how stellar emission bumps clear out completely when the full galaxy is integrated over, leaving only the narrow nebular component. Compare this to the FUSE data in Fig. 12, which we discuss in Sect. 4.2.