K2-19 is the second multi-planetary system discovered with K2 observations .
The system is composed of two Neptune size planets close to the 3:2 mean-motion resonance .
To better characterise the system we obtained two additional transit observations of K2-19b and five additional radial velocity observations .
These were combined with K2 data and fitted simultaneously with the system dynamics ( photo-dynamical model ) which increases the precision of the transit time measurements .
The higher transit time precision allows us to detect the chopping signal of the dynamic interaction of the planets that in turn permits to uniquely characterise the system .
Although the reflex motion of the star was not detected , dynamic modelling of the system allowed us to derive planetary masses of M _ { b } = 44 \pm 12 M _ { \oplus } and M _ { c } = 15.9 \pm 7.0 M _ { \oplus } for the inner and the outer planets respectively , leading to densities close to Uranus .
We also show that our method allows the derivation of mass ratios using only the 80 days of observations during the first campaign of K2 .