Context : The harvest of exoplanet discoveries has opened the area of exoplanets characterisation .
But this can not be achieved without a careful analysis of the host star parameters .

Aims : The system of HD 219134 hosts two transiting exoplanets and at least two additional non-transiting ones .
We revisit the properties of this system using direct measurements of the stellar parameters to investigate the composition of the two transiting exoplanets .

Methods : We used the VEGA/CHARA interferometer to measure the angular diameter of HD 219134 .
We also derived the stellar density from the transits light curves , which finally gives a direct estimate of the mass .
This allows us to infer the mass , radius and density of the two transiting exoplanets of the system .
We then use an inference model to obtain the internal parameters of these two transiting exoplanets .

Results : We measure a stellar radius , density and mass of R _ { \star } = 0.726 \pm 0.014 R _ { \odot } , \rho _ { \star } = 1.82 \pm 0.19 \rho _ { \odot } and M _ { \star } = 0.696 \pm 0.078 M _ { \odot } , respectively , with a correlation of 0.46 between R _ { \star } and M _ { \star } .
This new mass is smaller than that derived from the C2kSMO stellar evolutionary model , which provides a mass range of 0.755 - 0.810 ( \pm 0.040 ) M _ { \odot } .
Moreover , we find that planet b and c have smaller radii than previously estimated ( 1.500 \pm 0.057 and 1.415 \pm 0.049 R _ { \mathrm { p } } respectively ) , which clearly puts them out of the gap in the exoplanetary radii distribution , and validates their Super-Earth nature .
Planet b is more massive than planet c , but possibly less dense .
We investigate whether this could be caused by partial melting of the mantle , and find that tidal heating due to non zero eccentricity of planet b may be powerful enough .

Conclusions : The system of HD 219134 constitutes a very valuable benchmark for both stellar physics and exoplanetary science .
The characterisation of the stellar hosts , and in particular the direct determination of the stellar density , radius and mass , should be more extensively applied to provide accurate exoplanets properties and calibrate stellar models .