$\mathrm{O}$/$\mathrm{SO}$ Gauge Groups, $BC$ Quivers and $O3$ Planes

Abstract: D3-/D5-/NS5-brane systems with $O3$ orientifold planes realise 3d $\mathcal{N}=4$ gauge theories with orthogonal and symplectic gauge groups on the D3-brane worldvolume. Such setups have long contained an ambiguity regarding the global form of $D$-type gauge groups. This note offers a partial prescription for reading $\mathrm{O}(2k)$ and $\mathrm{SO}(2k)$ gauge nodes in orthosymplectic quivers using the presence of $\frac{1}{2}$D5-branes on the orientifolds bordering $\frac{1}{2}$NS5-brane intervals spanned by $O3^{-}$ planes. A set of identities are proposed relating the Coulomb branches of generic quivers under a Higgsing that relates $\mathrm{SO}(2k+1)$ and $\mathrm{O}(2k)$ gauge groups. A further prescription is conjectured regarding the action of $\frac{1}{2}$D5-branes on maximal $DC$-chains.