  The first UV F300W and F218W WFPC2 observations of the rich galaxy cluster
CL0939+4713 at z=0.41 are presented and discussed. UV/optical two-color and c-m
diagrams of the sources detected in the F300W waveband are constructed. Thanks
to pre-existing HST optical images of the same field a morphological
classification for the majority of these objects is also provided. Moreover,
taking advantage of recent redshift surveys along CL0939+4713 line of sight,
separate diagrams comparing the properties of galaxies belonging to the cluster
and to its close projected field are presented. Possible evolutionary effects
in the UV from z~0.4 to present time are investigated by comparing the
restframe [mid-UV-Optical] colors of galaxies in CL0939+4713 with balloon-borne
data of the Coma cluster, as well as by resorting to suitable galaxy evolution
models. Finally, current attempts to constrain the epoch of the UV-upturn onset
in evolved populations by means of HST UV observations are discussed.
