  We analyze systematics in the asteroseismological mass determination methods
in pulsating PG 1159 stars. We compare the seismic masses resulting from the
comparison of the observed mean period spacings with the usually adopted
asymptotic period spacings, and the average of the computed period spacings.
Computations are based on full PG1159 evolutionary models with stellar masses
ranging from 0.530 to 0.741 Mo that take into account the complete evolution of
progenitor stars. We conclude that asteroseismology is a precise and powerful
technique that determines the masses to a high internal accuracy, but it
depends on the adopted mass determination method. In particular, we find that
in the case of pulsating PG 1159 stars characterized by short pulsation
periods, like PG 2131+066 and PG 0122+200, the employment of the asymptotic
period spacings overestimates the stellar mass by about 0.06 Mo as compared
with inferences from the average of the period spacings. In this case, the
discrepancy between asteroseismological and spectroscopical masses is markedly
reduced when use is made of the mean period spacing instead of the asymptotic
period spacing.
