  Nitrogen oxides are thought to play a significant role as a nitrogen
reservoir and to potentially participate in the formation of more complex
species. Until now, only NO, N$_2$O and HNO have been detected in the
interstellar medium. We report the first interstellar detection of nitrous acid
(HONO). Twelve lines were identified towards component B of the low-mass
protostellar binary IRAS~16293--2422 with the Atacama Large
Millimeter/submillimeter Array, at the position where NO and N$_2$O have
previously been seen. A local thermodynamic equilibrium model was used to
derive the column density ($\sim$ 9 $\times$ 10$^{14}$ cm$^{-2}$ in a 0.5''
beam) and excitation temperature ($\sim$ 100 K) of this molecule. HNO, NO$_2$,
NO$^+$, and HNO$_3$ were also searched for in the data, but not detected. We
simulated the HONO formation using an updated version of the chemical code
Nautilus and compared the results with the observations. The chemical model is
able to reproduce satisfactorily the HONO, N$_2$O, and NO$_2$ abundances, but
not the NO, HNO, and NH$_2$OH abundances. This could be due to some thermal
desorption mechanisms being destructive and therefore limiting the amount of
HNO and NH$_2$OH present in the gas phase. Other options are UV
photodestruction of these species in ices or missing reactions potentially
relevant at protostellar temperatures.
