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We present the discovery and asteroseismic analysis of a new g-mode hot Bsubdwarf (sdB) pulsator, EC 21494-7018 (TIC 278659026), monitored in TESS firstsector using 120-second cadence. The light curve analysis reveals that EC21494-7018 is a sdB pulsator counting up to 20 frequencies associated withindependent g-modes. The seismic analysis singles out an optimal model solutionin full agreement with independent measurements provided by spectroscopy(atmospheric parameters derived from model atmospheres) and astrometry(distance evaluated from Gaia DR2 trigonometric parallax). Several keyparameters of the star are derived. Its mass (0.391 +/- 0.009 Msun) issignificantly lower than the typical mass of sdB stars, and suggests that itsprogenitor has not undergone the He-core flash, and therefore could originatefrom a massive (>2 Msun) red giant, an alternative channel for the formation ofhot B subdwarfs. Other derived parameters include the H-rich envelope mass(0.0037 +/- 0.0010 Msun), radius (0.1694 +/- 0.0081 Rsun), and luminosity(8.2+/-1.1 Lsun). The optimal model fit has a double-layered He+H compositionprofile, which we interpret as an incomplete but ongoing process ofgravitational settling of helium at the bottom of a thick H-rich envelope.Moreover, the derived properties of the core indicate that EC 21494-7018 hasburnt ~43% (in mass) of its central helium and possesses a relatively largemixed core (Mcore = 0.198 +/- 0.010 Msun), in line with trends alreadyuncovered from other g-mode sdB pulsators analysed with asteroseismology.Finally, we obtain for the first time an estimate of the amount of oxygen (inmass; X(O)core = 0.16 -0.05 +0.13) produced at this stage of evolution by anhelium-burning core. This result, along with the core-size estimate, is aninteresting constraint that may help to narrow down the still uncertainC12(alpha,gamma)O16 nuclear reaction rate.

TESS first look at evolved compact pulsators : Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018