Residual spectra of non-Blazhko RRab and RRc stars after the dominant pulsation frequency and its harmonics are prewhitened from the data. The top four panels show the Kepler RRab stars of Table 1, while the bottom five panels illustrate the spectra of RRc stars. The blue dotted and red dashed lines show the positions of the (prewhitened) harmonics and the possible positions of the additional frequencies, respectively. The frequency identification at the top of the figure refers to the RRc stars with f₆₈ frequencies (shown in the bottom five panels), but a similar structure is also seen in our RRab stars (in the top four panels).