Context : The TESS satellite was launched in 2018 to perform high-precision photometry from space over almost the whole sky in a search for exoplanets orbiting bright stars .
This instrument has opened new opportunities to study variable hot subdwarfs , white dwarfs , and related compact objects .
Targets of interest include white dwarf and hot subdwarf pulsators , both carrying high potential for asteroseismology .

Aims : We present the discovery and detailed asteroseismic analysis of a new g -mode hot B subdwarf ( sdB ) pulsator , EC 21494-7018 ( TIC 278659026 ) , monitored in TESS first sector using 120-second cadence .

Methods : The TESS light curve was analyzed with standard prewhitening techniques , followed by forward modeling using our latest generation of sdB models developed for asteroseismic investigations .
By simultaneously best-matching all the observed frequencies with those computed from models , we identified the pulsation modes detected and , more importantly , we determined the global parameters and structural configuration of the star .

Results : The light curve analysis reveals that EC 21494-7018 is a sdB pulsator counting up to 20 frequencies associated with independent g -modes .
The seismic analysis singles out an optimal model solution in full agreement with independent measurements provided by spectroscopy ( atmospheric parameters derived from model atmospheres ) and astrometry ( distance evaluated from Gaia DR2 trigonometric parallax ) .
Several key parameters of the star are derived .
Its mass ( 0.391 \pm 0.009 M _ { \odot } ) is significantly lower than the typical mass of sdB stars and suggests that its progenitor has not undergone the He-core flash ; therefore this progenitor could originate from a massive ( \gtrsim 2 M _ { \odot } ) red giant , which is an alternative channel for the formation of sdBs .
Other derived parameters include the H-rich envelope mass ( 0.0037 \pm 0.0010 M _ { \odot } ) , radius ( 0.1694 \pm 0.0081 R _ { \odot } ) , and luminosity ( 8.2 \pm 1.1 L _ { \odot } ) .
The optimal model fit has a double-layered He+H composition profile , which we interpret as an incomplete but ongoing process of gravitational settling of helium at the bottom of a thick H-rich envelope .
Moreover , the derived properties of the core indicate that EC 21494-7018 has burnt \sim 43 \% ( in mass ) of its central helium and possesses a relatively large mixed core ( M _ { { core } } = 0.198 \pm 0.010 M _ { \odot } ) , in line with trends already uncovered from other g-mode sdB pulsators analyzed with asteroseismology .
Finally , we obtain for the first time an estimate of the amount of oxygen ( in mass ; X ( { O } ) _ { { core } } = 0.16 { } _ { -0.05 } ^ { +0.13 } ) produced at this stage of evolution by an helium-burning core .
This result , along with the core-size estimate , is an interesting constraint that may help to narrow down the still uncertain ^ { 12 } { C } ( \alpha, \gamma ) ^ { 16 } { O } nuclear reaction rate .

Conclusions :