Lomb Scargle periodogram of vspan, including the window function of the data set. The grey bars at 0.3 height indicate the rotational frequency of the star as determined by Alina et al. (2012), i.e. 1.47 d^-1, as well as its harmonics (multiple of this frequency). Its width corresponds to the error bar. The lower bars at 0.15 and 0.02 height indicate the position of the ±1 and 2-day aliases with respect to the rotational frequency comb, respectively. The thin vertical black line indicates the same kind of frequency grid for a frequency of 1.77 d^-1 (see Sect. 3.3).