A CANDIDATE OPTICAL EINSTEIN RING

We present spectroscopic and imaging observations of a candidate optic al Einstein ring. The deflector is a luminous early-type galaxy, M(BT) = -20.9 + 5logh (q(0) = 0.1), at redshift z = 0.485. Superposed on th e otherwise normal optical spectrum of the galaxy is a strong emission feature at 5588 Angstrom. No other emission lines are evident over th e wavelength range lambda lambda 4456-7912 Angstrom. Narrow-band imagi ng of the system at the emission-line wavelength reveals a semicircula r are, radius r = 1.35 arcsec, centred on the peak of the galaxy surfa ce brightness distribution. The properties of the system can be reprod uced by a gravitational lens model in which the deflector has a consta nt mass-to-light ratio (corrected for evolution) of M/L(B(0)) similar to 20h and the source is an intrinsically small, but resolved, object at high redshift, either at z = 3.597 (if the line is Lya lambda 1216) , or at z = 2.607 (if the line is C IV lambda 1549). Alternatives to t he lensing explanation, in which the emission feature is associated wi th [O II] lambda 3727, with a velocity difference of 2900 km s(-1) rel ative to the absorption-line redshift of the galaxy, appear to be extr emely improbable. Unambiguous identification of the system as a gravit ational lens requires the detection of a second emission line, and wou ld confirm the system as the first Einstein ring to be discovered in t he optical.