Most massive stars are born in binary systems, with 70% expected to interact with a companion within their lifetimes. The consequences of these interactions for subsequent evolutionary stages and for the endpoints of stellar evolution are still not well understood, particularly for the red supergiant (RSG) phase: the final evolutionary stage before supernova for the majority of massive stars. To improve our understanding of RSG binary systems we propose to characterise hot companions around RSGs in the Small Magellanic Cloud (SMC) using HST ultra-violet STIS spectroscopy. Using a combination of newly acquired photometric and archival spectroscopic observations, we have selected a sample of 28 of the highest probability RSG binaries in the Small Magellanic Cloud. From stellar evolutionary models and the small number of confirmed RSG binaries in the Galaxy, we expect the majority of hot companions to be either main-sequence B-type stars or binary interaction products (e.g. from the merger of higher-order multiple components). These observations allow us to characterise SMC RSG binary systems by accurately determining masses of the hot companions to break degeneracies in orbital solutions. The results of this proposal represent a significant advancement in the characterisation of hot companions to RSGs, in orbital configurations and mass ranges that will ultimately result in double compact object binary systems.