The line of sight to the quasar B0218+357 , one of the most studied lensed systems , intercepts a z = 0.68 spiral galaxy , which splits its image into two main components A and B , separated by ca .
0.3 \arcsec , and gives rise to molecular absorption .
Although the main absorption component has been shown to arise in front of image A , it is not established whether some absorption from other velocity components is also occuring in front of image B .
To tackle this question , we have observed the HCO ^ { + } ( 2-1 ) absorption line during the commissioning phase of the new very extended configuration of the Plateau de Bure Interferometer , in order to trace the position of the absorption as a function of frequency .
Visibility fitting of the self-calibrated data allowed us to achieve position accuracy between \sim 12 and 80 mas per velocity component .
Our results clearly demonstrate that all the different velocity components of the HCO ^ { + } ( 2-1 ) absorption arise in front of the south-west image A of the quasar .
We estimate a flux ratio f _ { A } / f _ { B } = 4.2 _ { -1.0 } ^ { +1.8 } at 106 GHz .