10^{20}\,cm^{-2}$. On the other hand, assuming ${\mbox{$N_{\rm H}$}}= 5 \times 10^{20}\, \rm cm^{-2}$, as derived from the grand average spectrum, a composite function consisting of a white dwarf model atmosphere with at 36000K and of the same 12000K blackbody gives an equally satisfactory fit to the whole FOS spectrum. For a distance of 325pc the radius of the white dwarf is 4.9$\times 10^{8}$cm, in agreement with that of the hot blackbody component.

While the detection of the hot component is new, a comparison with previous spectral analysis of the optical and IR spin pulses observed in 1990 shows that the temperature of the cool component has not changed with time but instead it suffered a decrease in area by a factor of $\sim$ 1.5.

The sideband modulations
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The beat $\omega - \Omega$ modulation, although weak among the detected sidebands, shows an anti-phased behaviour between line fluxes and continuum (Fig.4a). The UV line fluxes, with average fractional amplitudes of $\sim 11\%$, show a minimum when the UV continuum is maximum. Their maximum shows a dip-like feature centered on the UV continumm minimum. Colour effects are also encountered, the far-UV pulses being stronger ($\sim 11\%$) than the near-UV ($\sim 6\%$) ones 