The orbital period was unevenly sampled since it is commensurable with the HST orbit. The G160L grating was operated with the blue digicon covering the range 1154–2508 Å at a resolution of 6.8 Å diode$^{-1}$ and with the 0.86" upper square aperture, supposed to be free from the 1500–1560 Å photocathode blemish which is known to affect the circular apertures (HST Data Handbook, 1997). A total of 797 spectra were collected, each with an effective exposure time of $\sim$ 18s.

The standard routine processing STScI pipeline applied to the data at the time of the observations revealed the presence of anomalous features when comparing the reduced spectra with the IUE data, and in particular in the regions 1500–1590Å and 1950–2010Å . The data were then re-processed using the STSDAS/CALFOS routine within IRAF using the latest reference files for sensitivity correction and appropriate aperture flatfield provided by the STScI Spectrograph Group in summer 1998. The calibrated spectra appear then free from the above features although the IUE fluxes are on average $\sim 1.1$ times larger than the FOS ones (see Fig.1). A check against systematic effects using spectra of standard stars observed with both IUE and FOS only confirms the known 