We present the frequency analysis of the multicolour time series photometry of the field RRd variable BS Comae. The large number of data points in each of the BV(RI)_c bands and the ~0.01 magnitude accuracy of the individual measurements allow us a high precision analysis of the properties of the combination frequencies due to nonlinear coupling. Through the combination of the frequency spectra in different colors we show that except for the components corresponding to the linear combinations of the two pulsation modes, there are no other components present above the millimagnitude amplitude level.
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