KIC 5950759: a high-amplitude ? Sct star with amplitude and frequency modulation near the terminal age main sequence

ABSTRACT Among the intermediate mass pulsating stars known as ? Sct is a subset of high-amplitude and predominantly radial-mode pulsators (HADS) stars. From more than 2000 observed by Kepler space mission, only two HADS were detected. We investigate perplexing these stars, KIC 5950759. study its variability using ground- space-based photometry, determine atmospheric parameters from spectroscopy perform asteroseismic modelling to constrain evolutionary stage. spectroscopy, we find that 5950759 metal-poor star, which in agreement with inferred metallicity needed reproduce pulsation mode frequencies non-adiabatic models. Furthermore, combine ground-based WASP measure linear change period order $\dot{P}/P \simeq 10^{-6}$ yr?1 for both fundamental first overtone radial modes across time base several years, at least orders magnitude larger predicted evolution models, largest measured star date. Our analysis indicates near short-lived contraction phase terminal-age main sequence, sub-solar making it candidate SX Phe star. unique object among thousands warrants further ascertain why are evolving remarkably faster stellar evolution.