Context :

Aims : Our aim is to confirm the interstellar detection of cyanic acid , HOCN , in the Galactic center clouds .
It has previously been tentatively detected only in Sgr B2 ( OH ) .

Methods : We used a complete line survey of the hot cores Sgr B2 ( N ) and ( M ) in the 3 mm range , complemented by additional observations carried out with the IRAM 30 m telescope at selected frequencies in the 2 mm band and towards four additional positions in the Sgr B2 cloud complex in the 2 and 3 mm bands .
The spectral survey was analysed in the local thermodynamical equilibrium approximation ( LTE ) by modeling the emission of all identified molecules simultaneously .
This allowed us to distinguish weak features of HOCN from the rich line spectrum observed in Sgr B2 ( N ) and ( M ) .
Lines of the more stable ( by 1.1 eV ) isomer isocyanic acid , HNCO , in these sources , as well as those of HOCN and HNCO towards the other positions , were analysed in the LTE approximation as well .

Results : Four transitions of HOCN were detected in a quiescent molecular cloud in the Galactic center at a position offset in ( R.A. , decl . )
by ( 20 ^ { \prime \prime } ,100 ^ { \prime \prime } ) from the hot core source Sgr B2 ( M ) , confirming its previous tentative interstellar detection .
Up to four transitions were detected toward five other positions in the Sgr B2 complex , including the hot cores Sgr B2 ( M ) , ( S ) , and ( N ) .
A fairly constant abundance ratio of \sim 0.3 - 0.8 % for HOCN relative to HNCO was derived for the extended gas components , suggesting a common formation process of these isomers .

Conclusions :