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We assume that the strong convection during core helium flash of low mass redgiant branch (RBG) stars excite waves that propagate to the envelope, and findthat the energy that these waves deposit in the envelope causes envelopeexpansion and brightening. We base our assumption and the estimate of the waveenergy on studies that explored such a process due to the vigorous coreconvection of massive stars just before they experience a core collapsesupernova explosion. Using the stellar evolutionary code MESA we find that thewave energy causes an expansion within few years by tens to hundreds solarradii. Despite the large brightening, we expect the increase in radius andluminosity to substantially enhance mass loss rate and dust formation. The dustshifts the star to become much redder (to the infrared), and the star mightactually become fainter in the visible. The overall appearance is of a faintred transient event that lasts for months to few years. We suggest that in somecases envelope expansion might lead stars that are about to leave the RGB toengulf exoplanets. The extended envelope has a smaller binding energy to adegree that allows planets of several Jupiter masses or more and brown dwarfsto survive the common envelope evolution. We suggest this scenario to accountfor the planet orbiting the white dwarf (WD) WD1856+534 (TIC 267574918) and forthe WD - brown dwarf binary system ZTFJ003855.0+203025.5.

Rapid expansion of red giant stars during core helium flash by waves propagation to the envelope and implications to exoplanets