  We present the discovery and 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. 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 +/- 0.009 Msun) is
significantly lower than the typical mass of sdB stars, and suggests that its
progenitor has not undergone the He-core flash, and therefore could originate
from a massive (>2 Msun) red giant, an alternative channel for the formation of
hot 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 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 ~43% (in mass) of its central helium and possesses a relatively large
mixed core (Mcore = 0.198 +/- 0.010 Msun), in line with trends already
uncovered from other g-mode sdB pulsators analysed 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
C12(alpha,gamma)O16 nuclear reaction rate.
