NEUTRAL HYDROGEN-21 CENTIMETER ABSORPTION AT Z=0.6847 TOWARD THE SMALLEST EINSTEIN RING

We detect neutral hydrogen 21 cm absorption toward the ''smallest Einstein ring,'' 0218 + 357, at a redshift of 0.6847. The implied H I column density is N(H I) = 4 x 10(18)(T(s)/f) cm-2, where T(s) is the spin temperature of the gas and f is the H I covering factor. The FWHM of the line based on a Gaussian fit is 43 km s-1, and the full width to the first null is about 75 km s-1. This observation demonstrates a new, potentially powerful method with which to determine lens redshifts in otherwise difficult circumstances. It also allows for the possibility of ''mapping'' the gravitational potential of the lens in a way independent of lens models.