Context. Recently, new strong lens candidates have been discovered within the COSMOS field. Here we present VLT/FORS1 spectroscopic follow-up observations and HST/WFPC2 imaging of the system COSMOS 5921+0638, which shows quadruply lensed images and a perfect Einstein ring. Aims. With the available observations of COSMOS 5921+0638 we study the nature of the system as well as its photometric, spectroscopic and physical properties. Methods. Using our VLT/FORS1 spectroscopy and Subaru/CFHT/HST imaging, we have extracted information on the system. We have done an environmental analysis and detailed analytical and grid-based mass modeling. Results. We have measured the redshift of the lensing galaxy in COSMOS 5921+0638 (zl=0.551±0.001) and of 9 additional galaxies in the field (5 of them at z∼0.35). The redshift of the source was inferred by identifying a candidate Lyα line at zs=3.14±0.05. The mass modeling reveals the requirement of a small external shear (γ=0.038), which is slightly larger than the lensing contribution expected due to galaxy groups in the line-of-sight obtained from the zCOSMOS optical group catalog (κgroups∼0.01 andγgroups∼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 isµ≈150. The measured mass-to-light ratio of the lensing galaxy within the Einstein ring is M/LB≈8.5±1.6
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