Context : Strong lens candidates have been newly identified within the COSMOS field .
We present VLT/FORS1 spectroscopic follow-up observations and HST/WFPC2 imaging of the system COSMOS 5921+0638 , which exhibits quadruply lensed images and a perfect Einstein ring .

Aims : We investigate the nature of COSMOS 5921+0638 by studying its photometric , spectroscopic and physical properties .

Methods : By analyzing our VLT/FORS1 spectroscopy and Subaru/CFHT/HST imaging of COSMOS 5921+0638 , we completed both an environmental analysis and detailed analytical and grid-based mass modeling to determine it properties .

Results : We measured the redshifts of the lensing galaxy in COSMOS 5921+0638 ( z _ { l } =0.551 \pm 0.001 ) and 9 additional galaxies in the field ( 5 of them at z \sim 0.35 ) .
The redshift of the lensed source was inferred by identifying a candidate Ly \alpha line at z _ { s } =3.14 \pm 0.05 .
The mass modeling reveals the requirement of a small external shear ( \gamma =0.038 ) , which is slightly larger than the lensing contribution expected by galaxy groups along the line-of-sight obtained from the zCOSMOS optical group catalog ( \kappa _ { groups } \sim 0.01 and \gamma _ { groups } \sim 0.005 ) .
The estimated time-delays between the different images are of the order of hours to half a week and the total magnification of the background source is \mu \approx 150 .
The measured mass-to-light ratio of the lensing galaxy within the Einstein ring is M / L _ { B } \approx 8.5 \pm 1.6 .
Anomalies are observed between the measured and expected flux ratios of the images of the background AGN .

Conclusions : Our analysis indicates that the ring and point-like structures in COSMOS 5921+0638 consist of a lensed high redshift galaxy hosting a low luminosity AGN ( LLAGN ) .
The observed flux ratio anomalies are probably due to microlensing by stars in the lensing galaxy and/or a combination of static phenomena .
Multi-epoch , multi-band space-based observations would allow us to differentiate between the possible causes of these anomalies , since static and/or dynamic variations could be identified .
Because of its short time-delays and the possibility of microlensing , COSMOS 5921+0638 is a promising laboratory for future studies of LLAGNs .