Illustration of the dependence of the frequency spectra on noise level (upper vs. lower panels) and on the method of data analysis (lighter shade: full time series model; darker shade: partial time series model). In this example the frequency of the test signal is 0.446 d^-1, very close to that of the systematics. Farther away from the frequency of the systematics the difference between the two methods is much smaller, even in the noiseless case.