Context : Shell galaxies are considered the debris of recent accretion/merging episodes .
Their high frequency in low density environments suggests that such episodes could drive the secular evolution for at least some fraction of the early-type galaxy population .

Aims : 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 .

Methods : 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 .

Results : 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 .

Conclusions :