Charged test particle scattering and effective one-body metrics with spin

Using recently developed techniques, we consider weak-field test particle scattering angle calculations in two distinct settings: Charged test particles in spacetimes of charged sources and effective one-body theory with spin. We present scattering angle calculations up to O(G(4)) of charged particles in the Kerr-Newman metric, including electromagnetic interactions up to second order in charge. Coulomb scattering is also discussed, and the well-known Darwin scattering formula is rederived by resummation. An effective one-body metric for a Kerr-Schwarzschild binary is constructed in a post-Minkowskian framework up to O(G(2)) and first order in spin. Facilitated by explicit scattering calculations, our approach is equivalent with existing literature through gaugelike transformations. Finally, we investigate if the Newman-Janis algorithm applied to an effective one-body metric of nonspinning binaries represents a binary system with spin.