TESS photometry of helium-rich hot subdwarfs: r modes in BD+37 degrees 442 and BD+37 degrees 1977

Transiting Exoplanet Survey Satellite photometry of the extremely helium-rich hot subdwarfs BD+37 degrees 442 and BD+37 degrees 1977 demonstrates multiperiodic low-amplitude variability with principal periods of 0.56 and 1.14 d, respectively, and with both first and second harmonics present. The light curves are not perfectly regular, implying additional periodic and/or nonperiodic content. Possible causes are examined, including the binary hypothesis originally introduced to explain X-ray observations, differentially rotating surface inhomogeneities, and pulsations. If the principal photometric periods correspond to the rotation periods, the stars are rotating at approximately 0.7 and 0.3 x break-up, respectively. Surface Rossby waves (r modes) therefore provide the most likely solution.