Top: example of a light curve with a strong alias. To guide the eye, sine waves for both the preferred period (P_rot = 0.8385 d, solid line) and the alias period (P_alias = 5.18 d, dashed line) are added to the data (black dots). The shorter period fits the data more closely as is shown by the fact that the simple sine connects nearly all data points. Bottom: corresponding CLEAN periodogram with both periods marked with vertical lines. The alias period is not located exactly at the peak in the periodogram due to the coarser resolution at longer periods.