EL CMi: Confirmation of triaxial pulsation theory

Triaxial pulsators are a recently discovered group of oscillating stars in close binary systems that pulsate around three axes at the same time. It has recently been theoretically shown that new types of pulsation modes, the tidally tilted standing (TTS) modes, can arise in these stars. We report the first detection of a quadrupole TTS oscillation mode in the pulsating component of the binary system EL CMi following an analysis of TESS space photometry. Two dipole oscillations around different axes in the orbital plane are present as well. In addition, we characterize the binary system using new radial velocity measurements and PHOEBE as well as simultaneous spectral energy distribution and light curve modeling. The pulsating primary component has properties typical of a δ Scuti star, but has accreted and is still accreting mass from its Roche-lobe filling companion. The donor star is predicted to evolve into a low-mass helium white dwarf. EL CMi demonstrates the potential of asteroseismic inferences of the structure of stars in close binaries before and after mass transfer and in three spatial dimensions.