Context :

Aims : We present an asteroseismological study on PG~0122+200 , the coolest known pulsating PG1159 ( GW Vir ) star .
Our results are based on an augmented set of the full PG1159 evolutionary models recently presented by Miller Bertolami & Althaus ( 2006 ) .

Methods : We perform extensive computations of adiabatic g -mode pulsation periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M _ { \odot } .
These models take into account the complete evolution of progenitor stars , through the thermally pulsing AGB phase and born-again episode .
We constrain the stellar mass of PG~0122+200 Â by comparing the observed period spacing with the asymptotic period spacing and with the average of the computed period spacings .
We also employed the individual observed periods to find a representative seismological model for PG~0122+200 .

Results : We derive a stellar mass of 0.626 M _ { \odot } from a comparison between the observed period spacing and the computed asymptotic period spacing , and a stellar mass of 0.567 M _ { \odot } by comparing the observed period spacing with the average of the computed period spacing .
We also find , on the basis of a period-fit procedure , an asteroseismological model representative of PG~0122+200 Â which is able to reproduce the observed period pattern with an average of the period differences of \overline { \delta \Pi _ { i } } = 0.88 s and a root-mean-square residual of \sigma _ { { } _ { \delta \Pi _ { i } } } = 1.27 s. The model has an effective temperature T _ { eff } = 81 500 K , a stellar mass M _ { * } = 0.556 M _ { \odot } , a surface gravity \log g = 7.65 , a stellar luminosity and radius of \log ( L _ { * } / L _ { \odot } ) = 1.14 and \log ( R _ { * } / R _ { \odot } ) = -1.73 , respectively , and a He-rich envelope thickness of M _ { env } = 1.9 \times 10 ^ { -2 } M _ { \odot } .
We derive a seismic distance d \sim 614 pc and a parallax \pi \sim 1.6 mas .
The results of the period-fit analysis carried out in this work suggest that the asteroseismological mass of PG~0122+200 Â could be \sim 6 - 20 \% lower than thought hitherto and in closer agreement ( to within \sim 5 \% ) with the spectroscopic mass .
This result suggests that a reasonable consistency between the stellar mass values obtained from spectroscopy and asteroseismology can be expected when detailed PG1159 evolutionary models are considered .

Conclusions :