Handler, GKurtz, D WRappaport, Saul ASaio, HFuller, J.Jones, DGuo, Z.Chowdhury, S.Sowicka, P.Alicavus, F KahramanStreamer, MargaretMurphy, Simon2022-06-232397-3366http://hdl.handle.net/1885/267707It has long been suspected that tidal forces in close binary stars could modify the orientation of the pulsation axis of the constituent stars. Such stars have been searched for, but until now never detected. Here we report the discovery of tidally trapped pulsations in the ellipsoidal variable HD 74423 in Transiting Exoplanet Survey Satellite (TESS) space photometry data. The system contains a δ Scuti pulsator in a 1.6 d orbit, whose pulsation mode amplitude is strongly modulated at the orbital frequency, which can be explained if the pulsations have a much larger amplitude in one hemisphere of the star. We interpret this as an obliquely pulsating distorted dipole oscillation with a pulsation axis aligned with the tidal axis. This is the first time that oblique pulsation along a tidal axis has been recognized. It is unclear whether the pulsations are trapped in the hemisphere directed towards the companion or in the side facing away from it, but future spectral measurements can provide the solution. In the meantime, the single-sided pulsator HD 74423 stands out as the prototype of a new class of obliquely pulsating stars in which the interactions of stellar pulsations and tidal distortion can be studied.This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. Funding for the TESS Asteroseismic Science Operations Centre is provided by the Danish National Research Foundation (grant agreement DNRF106), ESA PRODEX (PEA 4000119301) and Stellar Astrophysics Centre (SAC) at Aarhus University. Some of the observations reported in this paper were obtained with the Southern African Large Telescope (SALT). Polish participation in SALT is funded by grant MNiSW DIR/WK/2016/07. D.W.K. acknowledges financial support from the STFC via grant ST/M000877/1. M.S. is supported by an Australian Government Research Training Program (RTP) Scholarship. G.H., S.C., F.K.A. and P.S. acknowledge financial support by the Polish NCN grant 2015/18/A/ST9/00578. D.J. acknowledges support from the State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) and the European Regional Development Fund (FEDER) under grant AYA2017-83383-P. We thank the TESS team and staff and TASC/TASOC for their support of the present work and Allan R. Schmitt for making his light-curve examining software LcTools freely available. S.C. is grateful to C. Engelbrecht for introducing him to the use of the observing equipment. G.H. thanks E. Paunzen for helpful discussions on the spectra of λ Boötis stars. A.V. is a NASA Sagan Fellow.application/pdfen-AU© 2020 The authors202010.1038/s41550-020-1035-12021-08-01