Maximum of the χ² obtained in an epoch-folding search around the expected spin period as a function of the T^⋆ adopted to correct the time series for the delays induced by the orbital motion. In this figure we show the result of the final search performed on all the three observations for which the pulsation was detected after having corrected for the orbital motion using the value T^⋆ as estimated from Fig. 2. A T^⋆ step of 0.15 s was adopted and 200 T^⋆ were tried. The best-fit model, constant + Gaussian, is also shown (dashed curve).