B0218 + 35.7: a gravitationally lensed system with the smallest separation

Using data from the VLA, the newly enhanced MERUN and the European VLBI Network, we have established that the strong flat-spectrum radio source B0218 + 35.7 is a gravitationally lensed system. The overall image separation of 335 milliarcsec (mas) is one-third that of any other securely established lensed system. The structure of B0218 + 35.7 is simple. It has two bright flat-spectrum compact components, one of which lies inside and the other outside a steep-spectrum 'Einstein ring' whose diameter is the same as the separation of the bright components. The pectral and polarization properties of these bright components, together with their substructure revealed by European VLBI, leave no doubt that they are two images of a single source. Their small separation suggests that the mass responsible for lensing is an order of magnitude smaller than in other lenses and that the dearth of systems with subarcsecond separations in previous surveys may simply be due to observational selection. MERLIN observations at three epochs show that the ratio of the flux densities of the bright components has changed by ∼ 10 per cent. It will, therefore, be possible to determine the time delay arising from the difference between the two light paths. The B0218 + 35.7 system, with its two strong variable components plus a nearly circular Einstein ring, therefore fits the description by Kochanek of a system that will enable the long-recognized potential of gravitational lenses for the determination of H0 to be realized.