  Exoplanet science is now in its full expansion, particularly after the CoRoT
and Kepler space missions that led us to the discovery of thousands of
extra-solar planets. The last decade has taught us that UV observations play a
major role in advancing our understanding of planets and of their host stars,
but the necessary UV observations can be carried out only by HST, and this is
going to be the case for many years to come. It is therefore crucial to build a
treasury data archive of UV exoplanet observations formed by a dozen "golden
systems" for which observations will be available from the UV to the infrared.
Only in this way we will be able to fully exploit JWST observations for
exoplanet science, one of the key JWST science case.
