Contour levels of χ² obtained by varying both parameters b and Λ in the flux-adjusted phase fit. The contour levels are shown for χ_min² + 2.3 (blue) and χ_min² + 4.61 (red), corresponding to confidence levels of 68% and 90%, respectively, for a fit with two parameters (Lampton et al. 1976). The black dot indicates the values of b and Λ corresponding to the minimum χ², while the dashed black line represents the contour corresponding to the constant value given by the product b × Λ.