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Based on spectroscopic constraints derived from NLTE line formation, weexplore the likely range of stellar parameters (T_eff and log g) for thehyper-metal-poor (HMP) star HE 1327-2326. Combining the constraints from Balmerline profiles and the Ca I/II ionization equilibrium, a subgiant stage ofevolution is indicated. This result is further supported by spectrophotometricobservations of the Balmer jump. If a higher T_eff value was used (as favouredby some photometric calibrations), the spectroscopic analysis would indicate aturnoff-point stage of evolution. Using a stellar-structure code that treatsthe effects of atomic diffusion throughout the star in detail, we evolve alow-mass model star to reach the HR-diagram position of HE 1327-2326 afterroughly 13 Gyr. While the surface abundances are modified significantly (bymore than 1 dex for the case of uninhibited diffusion), such corrections cannot resolve the discrepancy between the abundance inferred from thenon-detection of the Li I resonance line at 6707 Angstroem and the WMAP-basedprimordial lithium abundance. As there are numerous processes that can destroylithium, any cosmological interpretation of a lower-than-expected lithiumabundance at the lowest metallicities will have to await sample sizes ofunevolved hyper-metal-poor stars that are one order of magnitude larger. Thesituation remains equally inconclusive concerning atomic-diffusion corrections.Here, attempts have to be made to better constrain internal mixing processes,both observationally and by means of sophisticated modelling. With constraintson additional mixing processes taken from a recent globular-cluster study, thelikeliest scenario is that HE 1327-2326's surface abundances have undergonemild depletion (of order 0.2 dex).

HE 1327-2326, an unevolved star with [Fe/H] < -5.0 III. Does Its Atmosphere Reflect Its Natal Composition?