a) Surface brightness profiles (SBPs) of I Zw 18 C in V (F555W), R (F606W) and I (F814W). The thick gray curve shows a fit to the V SBP for R^⋆  ≥  38 with the modified exponential fitting function Eq. (1) (modexp) for a core radius R_c = 2.4·α (dotted vertical line). The effective radius R_eff and the radius R₈₀ enclosing 80% of the total V emission are indicated. The dashed line shows a linear fit to the outermost exponential part of the V SBP for R^⋆  ≥  6′′, i.e. at the extremely faint (μ ≥ 27.6 mag/◻″) outskirts of the galaxy. The shadowed area corresponds to the 1σ uncertainties of the V SBP. b) Radial V − R and R − I color profiles of I Zw 18 C, derived from the SBPs in the upper panel. c) Comparison of the best-fitting modexp model in panel a) (thick gray curve) with the V SBPs of I Zw 18 C after partial removal of the brightest point sources (V _⋆ ; squares). Open circles show the SBP of the unresolved stellar emission (V_d), computed after complete removal of compact ( ≤ 05) sources with an unsharp masking technique. d) Comparison of the color profiles in panel b) (open and filled circles) with the (V − I)_d color profile (squares) of the unresolved stellar emission. The shadowed area depicts the 1σ uncertainties of the V − R profile.