  White dwarfs are well studied objects. The relative simplicity of their
physics allows to obtain very detailed models which can be ultimately compared
with their observed properties. Among white dwarfs there is a specific class of
stars, known as ZZ-Ceti objects, which have a hydrogen-rich envelope and show
periodic variations in their light curves. G117-B15A belongs to this particular
set of stars. The luminosity variations have been successfully explained as due
to g-mode pulsations. G117-B15A has been recently claimed to be the most stable
optical clock ever found, being the rate of change of its 215.2 s period very
small: \dot{P}= (2.3 +- 1.4)x10^{-15} s s^-1, with a stability comparable to
that of the most stable millisecond pulsars. The rate of change of the period
is closely related to its cooling timescale, which can be accurately computed.
In this paper we study the pulsational properties of G117-B15A and we use the
observed rate of change of the period to impose constraints on the axion
emissivity and, thus, to obtain a preliminary upper bound to the mass of the
axion. This upper bound turns out to be 4cos^{2}{\beta} meV at the 95%
confidence level. Although there are still several observational and
theoretical uncertainties, we conclude that G117-B15A is a very promising
stellar object to set up constraints on particle physics.
