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It has long been suspected that tidal forces in close binary stars couldmodify the orientation of the pulsation axis of the constituent stars. Suchstars have been searched for, but until now never detected. Here we report thediscovery of tidally trapped pulsations in the ellipsoidal variable HD 74423 inTESS space photometry data. The system contains a Delta Scuti pulsator in a1.6-d orbit, whose pulsation mode amplitude is strongly modulated at theorbital frequency, which can be explained if the pulsations have a much largeramplitude in one hemisphere of the star. We interpret this as an obliquelypulsating distorted dipole oscillation with a pulsation axis aligned with thetidal axis. This is the first time that oblique pulsation along a tidal axishas been recognized. It is unclear whether the pulsations are trapped in thehemisphere directed towards the companion or in the side facing away from it,but future spectral measurements can provide the solution. In the meantime, thesingle-sided pulsator HD 74423 stands out as the prototype of a new class ofobliquely pulsating stars in which the interactions of stellar pulsations andtidal distortion can be studied.

Tidally Trapped Pulsations in a close binary star system discovered by TESS