Upper panel: the GLS periodogram of the XO-2N light curve collected by the APACHE survey and the corresponding window function, with the highest peak translated to the frequency of the best significant peak found in the periodogram. Levels of different FAP are also indicated. Lower panel: the APACHE light curve of XO-2N folded at the best period found by GLS with superposed the best sinusoidal fit of semi-amplitude A = 0.0027 mag (solid red line).