  We investigate the statistical equilibrium of Co in the atmospheres of cool
stars, and the influence of NLTE and HFS (hyperfine splitting) on the formation
of Co lines and abundances. Significant departures from LTE level populations
are found for Co I, also number densities of excited states in Co II differ
from LTE at low metallicity. The NLTE abundance of Co in solar photosphere is
4.95 +/- 0.04 dex, which is in agreement with that in C I meteorites within the
combined uncertainties. The spectral lines of Co I were calculated using the
results of recent measurements of hyperfine interaction constants by UV Fourier
transform spectrometry. For Co II, the first laboratory measurements of
hyperfine structure splitting A and B factors were performed. A differential
abundance analysis of Co is carried out for 18 stars in the metallicity range
-3.12 < [Fe/H] < 0. The abundances are derived by method of spectrum synthesis.
At low [Fe/H], NLTE abundance corrections for Co I lines are as large as +0.6
>... +0.8 dex. Thus, LTE abundances of Co in metal-poor stars are severely
underestimated. The stellar NLTE abundances determined from the single UV line
of Co II are lower by ~0.5-0.6 dex. The discrepancy might be attributed to
possible blends that have not been accounted for in the solar Co II line and
its erroneous oscillator strength. The increasing [Co/Fe] trend in metal-poor
stars, as calculated from the Co I lines under NLTE, can be explained if Co is
overproduced relative to Fe in massive stars. The models of galactic chemical
evolution are wholly inadequate to describe this trend suggesting that the
problem is in SN yields.
