Program: GN-2024A-Q-109

The donors in ultracompact binary systems represent the highly-stripped innermost few percent of an evolved star, but their nature and behaviour has been unknown for the 50-year history of the field. We propose to obtain the first ever high-cadence NIR photometry of an ultracompact donor using Gemini. We will use this to decompose the NIR luminosity of the donor star from the other binary components (accretor and accretion disc), in order to measure the donor star temperature and the day/night temperature difference. The temperature will be combined with already-measured masses and radii to distinguish between competing formation channels and to model the future evolution of the binary. The day/night temperature profile will be used to test and refine atmospheric models. These observations will answer the long-standing question about the nature of ultracompact donors.