Aims . Stellar parameters and abundances of Na , Mg , Al , K , Ca , Sr , Ba , and Eu are determined for four very metal-poor ( VMP ) stars ( -2.66 \leq [ Fe/H ] \leq - 2.15 ) .
For two of them , HD 84937 and HD 122563 , the fraction of the odd isotopes of Ba derived for the first time .

Methods . Determination of an effective temperature , surface gravity , and element abundances was based on non-local thermodynamic equilibrium ( non-LTE ) line formation and analysis of high-resolution ( R \sim 60000 and 90000 ) high signal-to-noise ( S/N \geq 200 ) observed spectra .
A model atom for H i is presented .
An effective temperature was obtained from the Balmer H _ { \alpha } and H _ { \beta } line wing fits .
The surface gravity was calculated from the Hipparcos parallax if available and the non-LTE ionization balance between Ca i and Ca ii .
Based on the hyperfine structure affecting the Ba ii resonance line \lambda 4554 , the fractional abundance of the odd isotopes of Ba was derived from a requirement that Ba abundances from the resonance line and subordinate lines of Ba ii must be equal .

Results . For each star , non-LTE leads to a consistency of T _ { eff } from two Balmer lines and to a higher temperature compared to the LTE case , by up to 60 K. Non-LTE effects are important in spectroscopic determination of surface gravity from the ionization balance between Ca i and Ca ii .
For each star with a known trigonometric surface gravity , non-LTE abundances from the lines of two ionization stages , Ca i and Ca ii , agree within the error bars , while a difference in the LTE abundances consists of 0.23 dex to 0.40 dex for different stars .
Departures from LTE are found to be significant for all investigated atoms , and they strongly depend on stellar parameters .
For HD 84937 , the Eu/Ba ratio is consistent with the relative solar system r - process abundances , and the fraction of the odd isotopes of Ba , f _ { odd } , equals 0.43 \pm 0.14 .
The latter can serve as an observational constraint on r - process models .
The lower Eu/Ba ratio and f _ { odd } = 0.22 \pm 0.15 found for HD 122563 suggest that the s - process or the unknown process has contributed significantly to the Ba abundance in this star .