  We explain the nature of the electronic band gap and optical absorption
spectrum of Carbon - Boron Nitride (CBN) hybridized monolayers using density
functional theory (DFT), GW and Bethe-Salpeter equation calculations. The CBN
optoelectronic properties result from the overall monolayer bandstructure,
whose quasiparticle states are controlled by the C domain size and lie at
separate energy for C and BN without significant mixing at the band edge, as
confirmed by the presence of strongly bound bright exciton states localized
within the C domains. The resulting absorption spectra show two marked peaks
whose energy and relative intensity vary with composition in agreement with the
experiment, with large compensating quasiparticle and excitonic corrections
compared to DFT calculations. The band gap and the optical absorption are not
regulated by the monolayer composition as customary for bulk semiconductor
alloys and cannot be understood as a superposition of the properties of
bulk-like C and BN domains as recent experiments suggested.
