Context :

Aims : In white-light coronagraph images , cool prominence material is sometimes observed as bright patches in the core of coronal mass ejections ( CMEs ) .
If , as generally assumed , this emission is caused by Thomson-scattered light from the solar surface , it should be strongly polarised tangentially to the solar limb .
However , the observations of a CME made with the SECCHI/STEREO coronagraphs on 31 August 2007 show that the emission from these bright core patches is exceptionally low polarised .

Methods : We used the polarisation ratio method of Moran and Davila ( 2004 ) to localise the barycentre of the CME cloud .
By analysing the data from both STEREO spacecraft we could resolve the plane-of-the-sky ambiguity this method usually suffers from .
Stereoscopic triangulation was used to independently localise the low-polarisation patch relative to the cloud .

Results : We demonstrated for the first time that the bright core material is located close to the centre of the CME cloud .
We show that the major part of the CME core emission , more than 85 % in our case , is H \alpha radiation and only a small fraction is Thomson-scattered light .
Recent calculations also imply that the plasma density in the patch is 8 10 ^ { 8 } cm ^ { -3 } or more compared to 2.6 10 ^ { 6 } cm ^ { -3 } for the Thomson-scattering CME environment surrounding the core material .

Conclusions :