White-dwarf stars are the end product of stellar evolution for most stars in the Universe 1 . Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution 2,3 . Moreover, they are important chronometers for dating galactic stellar populations, and their mergers with other white dwarfs now appear to be responsible for producing the type Ia supernovae that are used as standard cosmological candles 4 . However, the internal structure of white-dwarf stars-in particular their oxygen content and the stratification of their cores-is still poorly known, because of remaining uncertainties in the physics involved in stellar modelling codes 5,6 . Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogendeficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data 7,8 coupled with asteroseismic sounding techniques 9,10 to determine the internal constitution of this star. We find that the oxygen content and extent of its core exceed the predictions of existing models of stellar evolution. The central homogeneous core has a mass of 0.45 solar masses, and is composed of about 86 per cent oxygen by mass. These values are respectively 40 per cent and 15 per cent greater than those expected from typical white-dwarf models. These findings challenge present theories of stellar evolution and their constitutive physics, and open up an avenue for calibrating white-dwarf cosmochronology 11 .Hydrogen-deficient DB-type white dwarfs are dying stars that have effective temperatures ranging from about 12,000 K to upwards of 35,000 K (ref. 12). They have undergone a late final flash as postasymptotic-giant-branch (post-AGB) stars, which freed them from their thin remaining hydrogen envelope [13][14][15] . Their deeper chemical stratification-as in any typical white dwarf-is otherwise determined by the rate of the nuclear reaction 12 C(α ,γ ) 16 O, which synthesizes oxygen from the ashes of the core-helium-burning phase, by means of mixing processes associated with convection and rotation as well as the number of thermal pulses that occur in evolved AGB stars 16 . The descendant DB white dwarfs must bear the signature of such processes in their core, as well as the imprint of the still-ongoing settling of carbon and oxygen in the surrounding helium-rich envelope.KIC08626021 (GALEX J192904.6+ 444708) is the first pulsating DB white dwarf (also known as the V777 Her class of variable stars) to be moni tored extensively by the Kepler spacecraft for its pulsation properties 8 . This star shows an oscillation spectrum composed of non-radial g-modes, which are potentially sensitive to the deep interior of the star 17 . Using an analysis of 23 months of Kepler highprecision photo metric data 18 , we exploit the eight well secured independent modes, which have periods ranging from 143.2 s to 376.1 s (Fig. 1, Extended Data Table 1 and Methods). These relatively short pulsation periods are-accordin...