  Winds of cool dwarfs are difficult to observe, with only a few M dwarfs
presenting observationally-derived mass-loss rates (Mdot), which span several
orders of magnitude. Close-in exoplanets are conveniently positioned in the
inner regions of stellar winds and can, thus, be used to probe the
otherwise-unobservable local properties of their host-stars' winds. Here, we
use local stellar wind characteristics observationally-derived in the studies
of atmospheric evaporation of the warm-neptune GJ436 b to derive the global
characteristics of the wind of its M-dwarf host. Using an isothermal wind
model, we constrain the stellar wind temperature to be in the range [0.36,0.43]
MK, with Mdot=[0.5,2.5] x 10^{-15} Msyn/yr. By computing the pressure balance
between the stellar wind and the interstellar medium, we derive the size of the
astrophere of GJ436 to be around 25 au, significantly more compact than the
heliosphere. We demonstrate in this paper that transmission spectroscopy,
coupled to planetary atmospheric evaporation and stellar wind models, can be a
useful tool for constraining the large-scale wind structure of planet-hosting
stars. Extending our approach to future planetary systems discoveries will open
new perspectives for the combined characterisation of planetary exospheres and
winds of cool dwarf stars.
