Context :

Aims : The discovery of the first active interstellar object 2I/Borisov provides an unprecedented opportunity to study planetary formation processes in another planetary system .
In particular , spectroscopic observations of 2I allow us to constrain the composition of its nuclear ices .

Methods : We obtained optical spectra of 2I with the 4.2 m William Herschel and 2.5 m Isaac Newton telescopes between 2019 September 30 and October 13 , when the comet was between 2.5 au and 2.4 au from the Sun .
We also imaged the comet with broadband filters on 15 nights from September 11 to October 17 , as well as with a CN narrow-band filter on October 18 and 20 , with the TRAPPIST-North telescope .

Results : Broadband imaging confirms that the dust coma colours ( B-V=0.82 \pm 0.02 , V-R=0.46 \pm 0.03 , R-I=0.44 \pm 0.03 , B-R=1.28 \pm 0.03 ) are the same as for Solar System comets .
We detect CN emission in all spectra and in the TRAPPIST narrow-band images with production rates between 1.6 \times 10 ^ { 24 } and 2.1 \times 10 ^ { 24 } molec/s .
No other species are detected .
We determine three-sigma upper limits for C _ { 2 } , C _ { 3 } , and OH production rates of 6 \times 10 ^ { 23 } molec/s , 2 \times 10 ^ { 23 } molec/s and 2 \times 10 ^ { 27 } molec/s , respectively , on October 02 .
There is no significant increase of the CN production rate or A ( 0 ) f \rho during our observing period .
Finally , we place a three-sigma upper limit on the Q ( C _ { 2 } ) /Q ( CN ) ratio of 0.3 ( on October 13 ) .
From this , we conclude that 2I is highly depleted in C _ { 2 } , and may have a composition similar to Solar System carbon-chain depleted comets .

Conclusions :