  We investigated the discrepancy between planetary mass determination using
the transit timing variations (TTVs) and radial velocities (RVs), by analysing
the multi-planet system Kepler-9. Despite being the first system characterised
with TTVs, there are several discrepant solutions in the literature, with those
reporting lower planetary densities being apparently in disagreement with
high-precision RV observations. To resolve this, we gathered HARPS-N RVs at
epochs that maximised the difference between the predicted RV curves from
discrepant solutions in the literature. We also re-analysed the full Kepler
data-set and performed a dynamical fit, within a Bayesian framework, using the
newly derived central and duration times of the transits. We compared these
results with the RV data and found that our solution better describes the RV
observations, despite the masses of the planets being nearly half that
presented in the discovery paper. We therefore confirm that the TTV method can
provide mass determinations that agree with those determined using
high-precision RVs. The low densities of the planets place them in the scarcely
populated region of the super-Neptunes / inflated sub-Saturns in the
mass-radius diagram.
