  The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum
Chromodynamics has as consequence the existence of axions, hypothetical weakly
interacting particles whose mass is constrained to be on the sub-eV range. If
these particles exist and interact with electrons, they would be emitted from
the dense interior of white dwarfs, becoming an important energy sink for the
star. Due to their well known physics, white dwarfs are good laboratories to
study the properties of fundamental particles such as the axions. We study the
general effect of axion emission on the evolution of helium-rich white dwarfs
and on their pulsational properties. To this aim, we calculate evolutionary
helium-rich white dwarf models with axion emission, and asses the pulsational
properties of this models. Our results indicate that the rates of change of
pulsation periods are significantly affected by the existence of axions. We are
able for the first time to independently constrain the mass of the axion from
the study of pulsating helium-rich white dwarfs. To do this, we use an
estimation of the rate of change of period of the pulsating white dwarf PG
1351+489 corresponding to the dominant pulsation period. From an
asteroseismological model of PG 1351+489 we obtain $g_{ae}<3.3\times10^{-13}$
for the axion-electron coupling constant, or $m_a\cos^2{\beta}\lesssim$ 11.5
meV for the axion mass. This constraint is relaxed to
$g_{ae}<5.5\times10^{-13}$ ($m_a\cos^2{\beta}\lesssim$ 19.5 meV), when no
detailed asteroseismological model is adopted for the comparison with
observations.
