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.