EL CMi: Confirmation of triaxial pulsation theory

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, PL-00-716 Warszawa, Poland
² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA
³ Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain
⁴ Departamento de Astrofísica, Universidad de la Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Citizen Scientist, c/o Zooniverse, Dept., of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Amateur Astronomer, Glendale, AZ 85308, USA
⁷ TAPIR, Mailcode 350-17, California Institute of Technology, Pasadena, CA 91125, USA
⁸ Centre for Space Research, North-West University, Mahikeng 2745, South Africa
⁹ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
¹⁰ Department of Physics, Gibbet Hill Road, University of Warwick, Coventry CV4 7AL, UK
¹¹ Yunnan Observatories, Chinese Academy of Sciences (CAS), Kunming 650216, PR China
¹² International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, PR China
¹³ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁴ SETI Institute, 189 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA

^⋆ Corresponding author: gerald@camk.edu.pl

Received: 27 June 2025
Accepted: 27 July 2025

Abstract

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.