a wavelength bin of $\sim$ 1.8Å , were then produced and fitted with a sinusoid. The resulting amplitudes define the rotational pulsed spectrum as $\rm F_{\lambda} = 2\,A_{\lambda}$. This spectrum (shown in the enlargement of the lower panel of Fig.3), has to be regarded as an upper limit to the modulated flux since no pre–whitening could be performed given the low S/N of each 780 light curves. This spectrum gives evidence of modulation not only in the main emission lines and  absorption but also in the weaker emission features identified in sect.3. Broad band UV continuum and optical photometric spin pulsed fluxes, obtained from the multi-frequency fit and reported in Fig.3, provide a correct description of the rotational pulsed energy distribution. A spectral fit to the broad band UV and optical spectrum, using a composite spectral function, consisting of two blackbodies, gives 37500$\pm$500K and 12000$\pm$400K ($\chi^{2}_{\rm red} = 0.91$). The projected fractional area of the hot component is $\sim$0.11A$_{\rm wd}$, while the cool one covers $\rm \sim 10.7\,A_{\rm wd}$, for $\rm R_{wd}=8\times 10^{8}\,cm$, and d=325pc (Paper1).

The FOS rotational pulsed spectrum shows the presence of a  absorption feature which gives a hydrogen column density of $\rm 8 \pm 2 \times 