MERLIN/VLA imaging of the gravitational lens system B0218+357

Gravitational lenses offer the possibility of accurately determining the Hu bble parameter (H-0) over cosmological distances, and B0218+357 is one of t he most promising systems for an application of this technique. In particul ar this system has an accurately measured time delay (10.5 +/- 0.4 d) and p reliminary mass modelling has given a value for H-0 of 69(-19)(+13) km s(-1 ) Mpc(-1). The error on this estimate is now dominated by the uncertainty i n the mass modelling. As this system contains an Einstein ring it should be possible to constrain the model better by imaging the ring at high resolut ion. To achieve this we have combined data from MERLIN and the Very Large A rray (VLA) at a frequency of 5 GHz. In particular MERLIN has been used in m ultifrequency mode in order to improve substantially the aperture coverage of the combined data set. The resulting map is the best that has been made of the ring and contains many new and interesting features. Efforts are cur rently underway to exploit the new data for lensing constraints using the L ensClean algorithm.