Context : 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 .

Aims : 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 .

Methods : 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 .

Results : 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 _ { \odot } range .
The inferred model-dependent stellar ages are found to range from 7.4 to 13.0 Gyr , with uncertainties of \sim 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 .

Conclusions : 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 ] \sim - 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 ) .