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the scientific goal of applying the doppler imaging technique to ap stars , is to derive observational constraints on the diffusion mechanism in the presence of a stellar magnetic field .
theoretical studies ( michaud et al , 1981 ; mgessier , 1984 ) which predict that certain elements raise , sink or move horizontally in magnetic stellar atmospheres need to be verified . this can only be achieved if an unambiguous correlation between the elemental abundance structures and the magnetic field geometry is obtained . however , two major problems are responsible for difficulties in reaching that goal .
firstly , doppler imaging can ( presently ) only be applied to stars which have a fairly weak magnetic field and hence only the variation of the integrated longitudinal field strength ( effective field , @xmath0 ) is usually determined .
this sets only weak limits to the ` real ' magnetic geometry . for objects with stronger fields
the spectral line profiles are severely influenced by zeeman broadening and are not mainly caused by abundance variations .
this limitation can be avoided by obtaining all four stokes parameters as a function of the stellar rotational phase .
then zeeman doppler imaging codes ( donati , 1995 ) can be applied to calculate the magnetic field strength and surface orientation in combination with the abundance distribution .
but these measurements of the stokes parameters are not yet available , though strong efforts are made to get them from polarimetry .
secondly , in the past , only a few elements were mapped , mostly silicon , rarely iron and chromium , not even all of them for each object .
furthermore , these maps were based on inversions from single ` unblended ' spectral lines , which are difficult to find , especially in cooler ap stars .
this introduced uncertainties leading to a questionable reliability of the derived maps and weakening their relevance to theoretical modelling .
considering this situation , we focused our programme on the production of abundance maps of up to five and more elements on the surface of each of our target stars . a brief report on the stars and
the progress of our project is given hereafter , followed by the presentation of results obtained for the magnetic ap star cu virginis .
as pointed out before , one crucial limitation of doppler imaging in the past was that only a few elements were mapped , mostly silicon and occasionally iron and chromium .
furthermore , the number of stars studied was not sufficient to draw general conclusions by relating the abundance maps to the approximate magnetic field structure , thereby providing new constraints on theory ( hatzes 1995 ) .
in particular , the abundance maps of elements which are pushed upwards by radiatively driven diffusion should look severely different from those of elements that have a tendency to sink .        the helium map ( fig .
1 ) is characterized by a dominant spot which appears to be at the approximate position of the positive magnetic pole ( phase 0.5 ) . only in the central part of this spot
does helium reach the solar abundance ; on all other parts of the stellar surface , it is depleted by about 1.5 dex .
this result confirms theoretical predictions by vauclair et al .
( 1991 ) which have been obtained by modelling helium abundance in main sequence magnetic stars by introducing a weak wind of ionized metals at the magnetic poles , where the field lines are vertical .    in total contrast to that ,
silicon ( fig .
2 ) is strongly depleted in the helium spot but overabundant on the remaining visible surface . in these regions
the magnetic field lines are mainly horizontal and as predicted by e.g. mgessier ( 1994 ) , silicon accumulates at the magnetic equator band . chromium and iron
have surface structures very similar to that of silicon , but both elements are less enhanced compared to their solar abundances
.    however , the magnesium ( fig .
3 ) distribution differs much from that of the other metals .
the main feature is a ring centred at the helium spot ( magnetic pole ) , but with an the overall abundance deficiency of about 1 dex and much less contrast than all other elements .
the helium spot distribution of cu vir was already found by hiesberger et al .
( 1995 ) , on the basis of data obtained in 1980 , which is an evidence for the high reliability of this result .
also , the silicon abundance structure confirms earlier results published by hatzes ( 1995 ) .
furthermore , there are strong indications that cu virginis has slowed down its rotation rate abruptly in the year 1985 ( pyper et al , 1998 ) .
this effect has not been found in any other ap star and no theoretical explanation can be given at the moment .    nevertheless , the doppler imaging results for cu virgins are a good demonstration of how important it is to obtain the surface distribution of different elements for the same star .
cu virginis is one of the very few objects for which a correlation of the abundance structures and the magnetic field geometry can be given with high significance .
furthermore , it is planned to obtain the abundance distributions for elements like c , n , o and some rare earth species .
this should complete the picture and may lead to a detailed theoretical modelling of diffusion processes interacting with the magnetic field of this star .
donati , j .- f . : 1995 , in _ stellar surface structure , iau symp .
176 _ , eds : k.g .
strassmeier and j.l .
linsky , kluwer , dordrecht , 53 hatzes , a.p . : 1995 , in _ stellar surface structure , iau symp .
176 _ , eds : k.g .
strassmeier and j.l .
linsky , kluwer , dordrecht , 305
doppler imaging , a technique which inverts spectral line profile variations of an ap star into a two - dimensional abundance maps , provides new observational constraints on diffusion mechanism in the presence of a global magnetic field . 
 a programme is presented here with the aim to obtain abundance distributions of at least five elements on each star , in order to study how different diffusion processes act under influence of a stellar magnetic field . 
 the importance of this multi - element approach is demonstrated , by presenting the abundance maps of helium , magnesium , silicon , chromium and iron for the magnetic b9psi star cu vriginis .    
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