Electron-impact excitation of diatomic hydride cations II: OH${}^{+}$ and SH${}^{+}$

R-matrix calculations combined with the adiabatic-nuclei-rotation and Coulomb-Born approximations are used to compute electron-impact rotational rate coefficients for two open-shell diatomic cations of astrophysical interest: the hydoxyl and sulphanyl ions, OH${}^{+}$ and SH${}^{+}$. Hyperfine resolved rate coefficients are deduced using the infinite-order-sudden approximation. The propensity rule $\mathrm{\Delta }F=\mathrm{\Delta }j=\mathrm{\Delta }N=\pm 1$ is observed, as is expected for cations with a large dipole moment. A model for OH${}^{+}$ excitation in the Orion Bar photon-dominated region (PDR) is presented which nicely reproduces Herschel observations for an electron fraction $x_e={10}^{-4}$ and an OH${}^{+}$ column density of $3\times {10}^{13}$~cm${}^{-2}$. Electron impact electronic excitation cross sections and rate coefficients for the ions are also presented.