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in a hierarchical evolutionary scenario , galaxies experience accretion / merging events during their lifetime . while early - type galaxies in nearby clusters appear ( homogeneously ) old , the field early - type galaxy population seems to contain genuinely , recently _ rejuvenated _ objects ( see e.g. * ? ? ? * ) . early - type galaxies showing fine structure , like shells , occupy a special position since they are believed to fill the gap between ongoing mergers and normal elliptical galaxies . shells are faint , sharp - edged stellar features characterizing @xmath0 16.5% of the field early - type galaxy population and avoiding cluster environments ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? two major scenarios for their origin have been proposed : _ a ) _ a weak interaction between galaxies @xcite , _ b ) _ merging / accretion events @xcite . in the former scenario , weak interaction can form long lasting azimuthally distributed shells through the interference of density waves producing a thick disc population of dynamically cold stars . however , this requires a cold thick disc , not found in ellipticals . in the merging / accretion events between galaxies of different masses ( mass ratios typically 1/10 - 1/100 , _ b ) _ scenario above ) shells are density waves formed from infalling stars from the companion during a minor merger . major merger can also produce shells @xcite . whatever the mechanism is , interaction / merging events seem to have played a significant role in the formation / evolution of the early - type class in the field and activated a new star formation event @xcite .    whether there is a link between shell galaxies , the early phases of merging processes , and the class of normal early - type galaxies is still an open question . to test the depth of the link , a multiwavelength approach is of paramount importance . the uv data probe the ongoing / recent star formation processes and study their distribution across the galaxy while the x - ray emission is connected to the past star formation and metal enrichment history of the bulk of the galaxy and may disclose hidden agn activity . we present _ galaxy evolution explorer _ ( _ galex _ ) , xmm-_newton _ optical monitor ( om ) and xmm - epic observations of ngc 474 , ngc 7070a , and eso 2400100 , three shell galaxies in the @xcite catalogue . [ cols= " < , < , < , < , < , < , < , < , < , < " , ]     [ table1 ] beside the presence of shells , clearly visible in all three , although more spectacular in ngc 474 , these galaxies share other properties : they are in low density environments and are interacting systems . however , the details of the interaction are quite different : companions are well separated in ngc 7070a @xcite and strongly interacting in ngc 474 as shown by h_i _ observations ( see e.g. * ? ? ? * ) . two nuclei are embedded and interacting within the eso 2400100 envelope @xcite . furthermore , dust is detected in ngc  474 and ngc  7070a , although only for the latter , there is kinematic evidence of an external acquisition . table  [ table1 ] gives the journal of the xmm - newton observations . x - ray data were treated with the standard routines provided by sas version 7.0 as suggested by xmm - newton science analysis system : users guide available on line . the x - ray observations of ngc 474 have been already discussed in @xcite .    during x - ray observations , shell galaxies were simultaneously imaged in the ultraviolet and optical bands with the optical monitor @xcite ) . observations have been performed using uvw1 and uvm2 bands which cover the ranges 245 - 320 nm and 205 - 245 nm , respectively . galaxies have been observed also in the u ( 300 - 390 nm ) and b ( 390 - 490 nm ) bands . the point spread function  fwhm  is @xmath02.0  in uvw1 , and 1.8  in uvm2 sampled with 0476@xmath10476 pixels . in order to complete the uv information about our galaxies we searched the _ galex _ archive and retrieved the data for ngc 474 . the _ galex _ mission and instruments are fully described in @xcite and @xcite . the spatial resolution of the images is @xmath045 and 60 fwhm in fuv ( 135 - 175 nm ) and nuv ( 175 - 275 nm ) respectively , sampled with 15@xmath115 pixels . we found _ galex _ data only for the ngc  474 , that was observed the october 5th , 2003 with an exposure of 1477 and 1647 seconds in nuv and fuv bands respectively . we aim at obtaining crucial information about the time at which the accretion / merging phenomenon has occured that can be derived from the far uv and optical colours . such information is important for a proper discussion of the x - ray data . according to the evidence provided by the sample of @xcite , the x - ray luminosity could evolve during the period that characterizes phases of the galaxy - galaxy encounter , the merging and the set - up of the merging towards a relaxed galaxy . figure  [ fig1 ] shows the isointensity contours from the adaptively smoothed xmm-_newton _ of ngc 7070a and eso 2400100 superposed on the dss plates . it is evident that the emission from these two galaxies has a significantly different morphology . in ngc 7070a the emission is rather compact both in the hard and soft bands . in eso 2400100 the soft band emission is significantly stronger than the hard one and extends further than in ngc 7070a . spectral results are in agreement with the spatial picture : the nuclear source of ngc 7070a is the dominant component while in eso 2400100 the emission is indicative of a low temperature plasma . @xcite showed that ngc 474 is at the bottom of the x - ray emission distribution of e galaxies : its x - ray emission is consistent with the low end of the expected emission from discrete sources . _ galex _ data of ngc  474 ( figure  [ fig2 ] ) show that the nuv emission extends to the galaxy bulge , while the fuv emission shows up only in the central regions of the galaxy . in the ngc 474 image obtained in the uvw1 and uvm2 filters outer shells are visible as in the optical image @xcite . both eso  2400100 ( figure  [ fig3 ] ) and ngc  7070a have extension similar to that of the optical image in the om - uvw1 and in the _ galex _ nuv filter . this implies that the uv emission comes from the same kind of stellar population . the fuv emission , more concentrated toward the nucleus is most likely dominated by the emission of different types of hot stars ( see also * ? ? ? * ) .    in figure  [ fig3 ] ( mid and left panels ) the uvw1 and uvm2 images show the two nuclei embedded in eso 2400100 . the northern nucleus is significantly bluer than the southern one . @xcite noticed that the two nuclei have a significantly different h@xmath2 line - strength indices ( 2.79 in the northern nucleus vs. 1.54 in the southern one ) suggesting a different stellar population composition . the accretion of faint galaxies seems one of the drivers of the secular evolution of galaxies in loose , poor groups . since shells are widely believed to be generated by an accretion event , shell galaxies potentially trace the typical secular evolution of early - type galaxies in such environments . furthermore , there is a growing evidence of multiple accretion events in the same galaxy . the complex shell system of ngc 474 is believed to be generated by two distinct accretion events @xcite . in this framework the double nucleus in eso  2400100 is an interesting and puzzling case at the same time . is the second nucleus the cause of the observed system of shells or another evidence of a new , on - going , accretion event ?
shell galaxies are considered the debris of recent accretion / merging episodes . their high frequency in low density environments suggest that such episodes could drive the secular evolution for at least some fraction of the early - type galaxy population . 
 we present here the preliminary results of ultraviolet and x - ray data for a sample of three shell galaxies , namely ngc  474 , ngc  7070a and eso  2400100 . 
 the far uv morphology and photometry are derived using the observations obtained with the _ galaxy evolution explorer _ and the xmm-_newton _ optical monitor . 
 we aim at investigating the _ rejuvenation _ processes in the stellar population using the uv information as well as at gaining information about the possible evolution with time of the x - ray emission due interaction / merging processes .