Time-resolved HST and IUE UV spectroscopy of the intermediate polar FO Aqr

Time resolved spectroscopy of the Intermediate Polar FO Aqr reveals the pre sence of multiple periodicities in the UV range. A strong orbital modulatio n dominates both continuum and emission line flux variabilities, while line velocity motions are only detected at the rotational frequency. A prominen t orbital periodicity is also observed in coordinated optical photometry, w here FO Aqr was previously found to be spin dominated. The spectral depende nce of the main periodicities shows the presence of multi-temperature compo nents in FO Aqr and for the first time a hot and a cool component in the ro tational modulation. From a comparison with previous UV and optical data ob tained in 1990, no spectral variations in the orbital and rotational variab ilities are detected, indicating no significant changes in the effects of X -ray illumination but rather a shrinking of the accretion curtain accompain ed by an increase in size of the thickened part of the accretion disc. Thes e observations, consistent with the recently discovered long term trend in the X-ray pulsation amplitudes, independently confirm a change in the accre tion mode in FO Aqr, which switched from a disc-fed into a disc-overflow st ate, likely triggered by mass accretion variations.