Schematic illustration of the effect that the slight spatial displacement between the [Oiii]λλ4959, 5007 and the Hα emission lines across an expanding shell of ionized gas might have on the V and R photometry of stars in its close vicinity. The shell, of thickness 100 pc ( ≈ 1′′at the distance to I Zw 18), comprises an inner and outer zone with, respectively, higher and lower [Oiii]/Hα ratios (cf. the discussion related to Figs. 6 and 9).