  The discovery of multiple transiting planetary systems offers new
possibilities for characterising exoplanets and understanding their formation.
The Kepler-9 system contains two Saturn-mass planets, Kepler-9b and 9c. Using
evolution models of gas giants that reproduce the sizes of known transiting
planets and accounting for all sources of uncertainties, we show that Kepler-9b
(respectively 9c) contains $45^{+17}_{-12}$\,\mearth\ (resp.
$31^{+13}_{-10}$\,\mearth) of hydrogen and helium and $35^{+10}_{-15}$\,\mearth
(resp. $24^{+10}_{-12}$\,\mearth) of heavy elements. More accurate constraints
are obtained when comparing planets 9b and 9c: the ratio of the total mass
fractions of heavy elements are $Z_{\rm b}/Z_{\rm c}=1.02\pm 0.14$, indicating
that, although the masses of the planets differ, their global composition is
very similar, an unexpected result for formation models. Using evolution models
for super-Earths, we find that Kepler-9d must contain less than 0.1% of its
mass in hydrogen and helium and predict a mostly rocky structure with a total
mass between 4 and 16\,\mearth.
