Gravitational-darkening slope ♭₁(i) against the rotational ratio η. The colours identify the values of the differential parameter α, and the line-styles are for the inclination angles. In a) to c) the slope ♭₁(i) was calculated with Υ(θ) = cos^kθ and from d) to f) b₁(i) was obtained with Υ(θ) = sin^kθ. As throughout in this work, rigid rotation (α = 0) is identified with green lines.