  Measurement of the time delay between multiple images of a gravitational lens
system is potentially an accurate method of determining the Hubble constant
over cosmological distances. One of the most promising candidates for an
application of this technique is the system B0218+357 which was found in the
Jodrell Bank/VLA Astrometric Survey (JVAS). This system consists of two images
of a compact radio source, separated by 335 milliarcsec, and an Einstein ring
which can provide a strong constraint on the mass distribution in the lens. We
present here the results of a three-month VLA monitoring campaign at two
frequencies. The data are of high quality and both images show clear variations
in total flux density, percentage polarization and polarization position angle
at both frequencies. The time delay between the variations in the two images
has been calculated using a chi-squared minimization to be 10.5+/-0.4 days at
95 per cent confidence, with the error being derived from Monte-Carlo
simulations of the light curves. Although mass modelling of the system is at a
preliminary stage, taking the lensing galaxy to be a singular isothermal
ellipsoid and using the new value for the time delay gives a value for the
Hubble constant of 69+13/-19 km/s/Mpc, again at 95 per cent confidence.

Astrophysics