  Very metal-poor halo stars are the best candidates for being among the oldest
objects in our Galaxy. Samples of halo stars with age determination and
detailed chemical composition measurements provide key information for
constraining the nature of the first stellar generations and the
nucleosynthesis in the metal-poor regime.} Age estimates are very uncertain and
are available for only a small number of metal-poor stars. Here we present the
first results of a pilot program aimed at deriving precise masses, ages and
chemical abundances for metal-poor halo giants using asteroseismology, and
high-resolution spectroscopy. We obtained high-resolution UVES spectra for four
metal-poor RAVE stars observed by the K2 satellite. Seismic data obtained from
K2 light curves helped improving spectroscopic temperatures, metallicities and
individual chemical abundances. Mass and ages were derived using the code
PARAM, investigating the effects of different assumptions (e.g. mass loss,
[alpha/Fe]-enhancement). Orbits were computed using Gaia DR2 data. {The stars
are found to be "normal" metal-poor halo stars (i.e. non C-enhanced), with an
abundance pattern typical of old stars (i.e. alpha and Eu-enhanced), and with
masses in the 0.80-1.0 M_sun range. The inferred model-dependent stellar ages
are found to range from 7.4 to 13.0 Gyr, with uncertainties of ~ 30%-35%. We
also provide revised masses and ages for metal-poor stars with Kepler seismic
data from APOGEE survey and a set of M4 stars. {The present work shows that the
combination of asteroseismology and high-resolution spectroscopy provides
precise ages in the metal-poor regime. Most of the stars analysed in the
present work (covering the metallicity range of [Fe/H] ~ -0.8 to -2 dex), are
very old >9 Gyr (14 out of 19 stars ), and all of them are older than > 5 Gyr
(within the 68 percentile confidence level).
