We employ a state-of-the-art asteroseismological model of G117-B15A, the archetype of the H-rich atmosphere (DA) white dwarf pulsators (also known as DAV or ZZ Ceti variables), and use the most recently measured value of the rate of period change for the dominant mode of this pulsating star to derive a new constraint on the mass of axion, the still conjectural non-barionic particle considered as candidate for dark matter of the Universe. Assuming that G117-B15A is truly represented by our asteroseismological model, and in particular, that the period of the dominant mode is associated to a pulsation g-mode trapped in the H envelope, we find strong indications of the existence of extra cooling in this star, compatible with emission of axions of mass m_a \cos^2 β = 17.4^{+2.3}_{-2.7} meV. ; Comment: 9 pages, 5 figures and 3 tables. Accepted for publication in MNRAS