  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.
