Binary quasars are extremely rare objects , used to investigate clustering on very small scales at different redshifts .
The cases where the two quasar components are gravitationally bound , known as physical binary quasars , can also exhibit enhanced astrophysical activity and therefore are of particular scientific interest .
Here we present the serendipitous discovery of a physical pair of quasars with an angular separation of \Delta \theta = ( 8.76 \pm 0.11 ) arcsec .
The redshifts of the two quasars are consistent within the errors and measured as z = ( 1.76 \pm 0.01 ) .
Under the motivated assumption that the pair does not arise from a single gravitationally lensed quasar , the resulting projected physical separation was estimated as ( 76 \pm 1 ) kpc .
For both targets we detected Si iv , C iv , C iii ] , and Mg ii emission lines .
However , the two quasars show significantly different optical colours , one being among the most reddened quasars at z > 1.5 and the other with colours consistent with typical quasar colours at the same redshift .
Therefore it is ruled out that the sources are a lensed system .
This is our second serendipitous discovery of a pair of two quasars with different colours , having a separation \lesssim 10 arcsec , which extends the very limited catalogue of known quasar pairs .
We ultimately argue that the number of binary quasars may have been significantly underestimated in previous photometric surveys , due to the bias arising from paired quasars with very different colours .