and lagging by $\sim$ 0.2 in phase the near-UV. The trend of a decrease in the amplitudes at increasing wavelengths is confirmed in the optical where an upper limit of $\sim$ 2$\%$ can be set to the fractional amplitudes.

A similar colour effect is also observed in the $\omega + \Omega$ sideband pulsation with fractional amplitudes ranging from $\sim 20\%$ in the far-UV to $\sim 7\%$ in the near-UV, displaying a broadening of the maximum towards longer wavelengths (Fig.4b).

The strong $\omega - 2\,\Omega$ pulsation only shows a wavelength dependence in the fractional amplitudes, being in the far-UV $\sim 26 \%$ and decreasing to $\sim 12\%$ in the near-UV (Fig.4c) and to $\sim 2-3\%$ (upper limit) in the optical.

The modulated spectra at the three frequencies have been derived following the same procedure as for the rotational pulsed spectrum. The anti-phased behaviour of the emission lines is seen in the beat pulsed spectrum, where these are seen as absorption features, except for residuals in the CIV and HeII lines (Fig4d, bottom panel). The modulation spectra at $\omega - 2\Omega$ and $\omega +\Omega$ frequencies show weak emissions at SiIV, CIV and HeII indicating a marginal variability at these frequencies. The UV 