HST orbital frequencies. These are removed by the method as shown by the residuals. Then a five (four) frequency composite sinusoidal function for each spectral band (emission line) has been used and the derived amplitudes, reported in Table2, have been compared with the average power in the DFTs of the residuals ($\sigma$) in the range of frequencies of interest (i.e. $\nu \la$ 1.4mHz), (column 8). While the signals at $\omega-2\,\Omega$ (continuum) and at $2\Omega$ (emission lines) on average fulfil a 4 $\sigma$ criterium, the other sidebands are between between 2.1 and 3.5 $\sigma$.

A further check has been performed using the CLEAN algorithm (Roberts et al. 1987) which removes the windowing effects of the HST orbit. The CLEANED power spectra, adopting a gain of 0.1 and 500 iterations, indeed reveal the presence of the $\omega -2\Omega$ and $\omega - \Omega$ sidebands in the continuum and the $2\Omega$ in the emission lines. The lack of significant power at the other frequencies is consistent with the previous analysis. Hence, these weakly active frequencies will be considered with some caution in this analysis.

  ------------------------- ------------- ------------- ------------- ----------- ------------- ----------- ------- -----------
                                                                                                                    
                                                                                                                    
                                                                                                                    
                                                                                                                    
  [**1265–1275**]{}             1.730(52)                   0.893(51)   0.913(72)     0.391(55)   0.693(72)   0.181 
  [**1425–1450**]{}             1.784(42)                   0.867(43)   0.694(59)     