Nondimensional factor ζ (on the ordinate) related to the periodic part of the rotation frequency change ∆ω (see the text), expressed as a function of the heliocentric distance (at the abscissa). The sense of motion is clarified by the arrows, which imply that the upper branch of the curve corresponds to the motion from perihelion to aphelion, and vice versa for the lower branch of the curve. This prediction corresponds to the specific case of (1862) Apollo; the secular part with the υ value determined in Sect. 2.1 has been subtracted from the rotation-rate ω(t) evolution and used for the normalization of the ζ function. The high eccentricity (⋍ 0.56) of the Apollo orbit implies that the amplitude of the effect is not too small. The red symbols indicate the epochs along the orbits for which light-curve observations are available. Some of the observations were taken during close encounters on extreme values of phase angle exceeding 90° at heliocentric distances smaller than 1 au (indicated by the vertical dashed line). However, Apollo is large enough to also be easily observable near the aphelion of its orbit at a heliocentric distance of 2.29 au (see also Table A.2).