continuum energy distributions of these variabilities are best represented by power laws $\rm F_{\lambda} \propto \lambda ^{-\alpha}$ with spectral index $\alpha \sim 0.8-1.7$, rather than blackbodies (20000 – 25000K), possibly suggesting that more than one component is acting. Given the low level of confidence of the sideband variabilities it is not possible to derive further information.

The orbital variability
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The UV and optical orbital modulations have been investigated folding the FOS continuum broad band and emission line fluxes in 28 orbital phase bins. The light curves have been prewhitened by the other active frequencies using the results of the multi-frequency fits. For the IUE data, continuum broad band and emission line flux measures have been performed on each SWP and LWP spectrum. Three broad bands have been selected in each spectral range, five of them coinciding with the FOS selected bands and a sixth one in the range $\lambda$ 2900–2985. The contribution of the spin pulsation, as derived from the multi-frequency fit has been removed. The best fit blackbody spin pulsed spectrum has been used to allow prewhitening in the range $\lambda$2900–2985. The FOS and IUE broad band continuum fluxes in the far-UV, mid-UV and near-UV as well as 