We report the discovery of a partial Einstein ring of radius 1 . ^ { \prime \prime } 48 produced by a massive ( and seemingly isolated ) elliptical galaxy .
The spectroscopic follow-up at the VLT reveals a 2 L _ { \star } galaxy at z = 0.986 , which is lensing a post-starburst galaxy at z = 3.773 .
This unique configuration yields a very precise measure of the mass of the lens within the Einstein radius , ( 8.3 \pm 0.4 ) 10 ^ { 11 } h _ { 70 } ^ { -1 } M _ { \odot } .
The fundamental plane relation indicates an evolution rate of d \log ( M / L ) _ { \mathrm { B } } / dz = -0.57 \pm 0.04 , similar to other massive ellipticals at this redshift .
The source galaxy shows strong interstellar absorption lines indicative of large gas-phase metallicities , with fading stellar populations after a burst .
Higher resolution spectra and imaging will allow the detailed study of an unbiased representative of the galaxy population when the universe was just 12 % of its current age .