Results of Monte-Carlo simulations to estimate the systematic uncertainties of the 2D decomposition with GALFIT for HE 2158 − 0107. The upper panels present the difference in simulated and observed host magnitudes (Δm_host) as a function of the simulated host magnitude for five different effective radii assuming a de Vaucouleurs surface brightness profile. Detection limits (3σ) in host brightness for the different effective radii are indicated by the solid vertical lines in their corresponding colour. The lower panels show the ratio of the observed over the simulated effective radii as a function of the simulated host brightness.