  We present XMM-Newton X-ray observations of two shell galaxies, NGC 7070A and
ESO 2400100, and far UV observations obtained with the Optical Monitor for
these and for an additional shell galaxy, NGC 474, for which we also have near
and far UV data from GALEX. We aim at gaining insight on the overall evolution
traced by their star formation history and by their hot gas content. The X-ray
and the far UV data are used to derive their X-ray spatial and spectral
characteristics and their UV luminosity profiles. We use models developed ad
hoc to investigate the age of the last episode of star formation from the (UV -
optical) colors and line strength indices. The X-ray spatial and spectral
analysis show significant differences in the two objects. A low luminosity
nuclear source is the dominant component in NGC 7070A log L_X=41.7 erg s^{-1}
in the 2-10 keV band. In ESO 2400100, the X-ray emission is due to a low
temperature plasma with a contribution from the collective emission of
individual sources. In the Optical Monitor image ESO 2400100 shows a double
nucleus, one bluer than the other. This probably results from a very recent
star formation event in the northern nuclear region. The extension of the UV
emission is consistent with the optical extent for all galaxies, at different
degrees of significance in different filters. The presence of the double
nucleus, corroborated by the (UV - optical) colors and line strength indices
analysis, suggests that ESO 2400100 is accreting a faint companion. We explore
the evolution of the X-ray luminosity during accretion processes with time. We
discuss the link between the presence of gas and age, since gas is detected
either before coalescence or several Gyr (>3) after (Abridged).
