Geometrical deformation of stellar surfaces produced by differential rotation laws given by Eqs. (4) and (5). Panel a: Stellar geometries calculated using Υ(θ) = cos^kθ, rotational ratio η = 0.8, differential rotation parameters α = − 0.5, 0.0 (solid body rotation) and 0.5, and powers k = 0.5, 2.0 and 6.0. Panel b: Calculations done with Υ(θ) = sin^kθ and the same values of η and powers k, but for α′ = − α/ (1 + α). In all cases the colours identify the values of the power k, while the line-styles characterize the values of the differential-rotation parameter α, and the corresponding α′.