  White dwarfs are the end-product of the lifes of intermediate- and low-mass
stars and their evolution is described as a simple cooling process. Recently,
it has been possible to determine with an unprecedented precision their
luminosity function, that is, the number of stars per unit volume and
luminosity interval. We show here that the shape of the bright branch of this
function is only sensitive to the averaged cooling rate of white dwarfs and we
propose to use this property to check the possible existence of axions, a
proposed but not yet detected weakly interacting particle. Our results indicate
that the inclusion of the emission of axions in the evolutionary models of
white dwarfs noticeably improves the agreement between the theoretical
calculations and the observational white dwarf luminosity function. The best
fit is obtained for m_a cos^2 \beta ~ 5 meV, where m_a is the mass of the axion
and cos^2 \beta is a free parameter. We also show that values larger than 10
meV are clearly excluded. The existing theoretical and observational
uncertainties do not allow yet to confirm the existence of axions, but our
results clearly show that if their mass is of the order of few meV, the white
dwarf luminosity function is sensitive enough to detect their existence.
