  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. 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. 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$0.03) are the same as for Solar System comets. We detect CN
emission in all spectra and in the TRAPPIST narrow-band images with production
rates between 1.6$\times10^{24}$ and 2.1$\times10^{24}$ molec/s. No other
species are detected. We determine three-sigma upper limits for C$_2$, C$_3$,
and OH production rates of 6$\times10^{23}$ molec/s, 3$\times10^{23}$ molec/s
and 2$\times10^{27}$ 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.
