Program: GS-2020B-FT-202

Pulsating stars offer the unique opportunity to directly measure stellar masses and radii. Recently, we tripled the number of radial mode hot subdwarf pulsators, and 16 members are known today. Hot subdwarfs are blue stars, and reside in an unusual part of the Hertzsprung Russell diagram below the main sequence. This has led to a variety of suggestions on their evolutionary history with very different predictions for their masses, radii and evolutionary stage. Most scenarios include evolution in a compact binary system with at least one phase of mass transfer. Observational data of these hot subdwarf radial mode pulsators is very sparse, even though their pulsations provide the unique opportunity to measure masses and radii directly. Here we propose to obtain phase-resolved spectra with Gemini/GMOS-S for two members of that class to measure masses and radii using the Baade-Wesselink method and the stellar evolution code MESA to constrain the evolutionary history of these unique radial mode pulsators. If the observations are successful we anticipate a larger follow-up campaign containing a larger sample of hot subdwarf radial mode pulators.