Upper panel: the RV variation produced by a single equatorial spot on HD 189733 vs. the rotation phase for different values of the parameter C_d according to the different linestyles: C_d = 0.0 (no temperature effect on the line profile): solid line; C_d = 0.2: dotted line; and C_d = 0.4: dashed line. Lower panel: the corresponding modulation of the optical flux vs. the rotation phase. Note that the three plots overlap because the modulation is independent of C_d (see the text).