Left: CEP-1998 as an example of a twin-peak target, for which secondary periods are found after pre-whitening the light curve using the primary period. Right: CEP-1543 as an example from the control group (no twin peaks). Colors trace observation date, increasing from yellow to blue. Top panels: phase-folded (with primary period) OGLE-III I-band light curve. Center panels: residuals after subtracting Fourier series model. Bottom panels: periodogram of the residuals shown in the center panel. Pink vertical lines at 0, 1, … d⁻¹ indicate parasite frequencies due to residual trends in the time series. The solid light blue vertical line indicates the OGLE catalog (primary) frequency. The dashed light blue lines correspond to its ± 1, 2, … aliases.