  The NASA Kepler space telescope has detected solar-like oscillations in
several hundreds of single stars, thereby providing a way to determine precise
stellar parameters using asteroseismology. In this work, we aim to derive the
fundamental parameters of a close triple star system, HD 188753, for which
asteroseismic and astrometric observations allow independent measurements of
stellar masses. We used six months of Kepler photometry available for HD 188753
to detect the oscillation envelopes of the two brightest stars. For each star,
we extracted the individual mode frequencies by fitting the power spectrum
using a maximum likelihood estimation approach. We then derived initial guesses
of the stellar masses and ages based on two seismic parameters and on a
characteristic frequency ratio, and modelled the two components independently
with the stellar evolution code CESTAM. In addition, we derived the masses of
the three stars by applying a Bayesian analysis to the position and
radial-velocity measurements of the system. Based on stellar modelling, the
mean common age of the system is $10.8 \pm 0.2\,$Gyr and the masses of the two
seismic components are $M_A =$ $0.99 \pm 0.01\,M_\odot$ and $M_{Ba} =$ $0.86
\pm 0.01\,M_\odot$. From the mass ratio of the close pair, $M_{Bb}/M_{Ba} =
0.767 \pm 0.006$, the mass of the faintest star is $M_{Bb} =$ $0.66 \pm
0.01\,M_\odot$ and the total seismic mass of the system is then $M_{syst} =$
$2.51 \pm 0.02\,M_\odot$. This value agrees perfectly with the total mass
derived from our orbital analysis, $M_{syst} =$
$2.51^{+0.20}_{-0.18}\,M_\odot$, and leads to the best current estimate of the
parallax for the system, $\pi = 21.9 \pm 0.2\,$mas. In addition, the minimal
relative inclination between the inner and outer orbits is $10.9^\circ \pm
1.5^\circ$, implying that the system does not have a coplanar configuration.
