Context : Observations of transiting extrasolar planets are of key importance to our understanding of planets because their mass , radius , and mass density can be determined .
These measurements indicate that planets of similar mass can have very different radii .
For low-density planets , it is generally assumed that they are inflated owing to their proximity to the host-star .
To determine the causes of this inflation , it is necessary to obtain a statistically significant sample of planets with precisely measured masses and radii .

Aims : The CoRoT space mission allows us to achieve a very high photometric accuracy .
By combining CoRoT data with high-precision radial velocity measurements , we derive precise planetary radii and masses .
We report the discovery of CoRoT-19b , a gas-giant planet transiting an old , inactive F9V-type star with a period of four days .

Methods : After excluding alternative physical configurations mimicking a planetary transit signal , we determine the radius and mass of the planet by combining CoRoT photometry with high-resolution spectroscopy obtained with the echelle spectrographs SOPHIE , HARPS , FIES , and SANDIFORD .
To improve the precision of its ephemeris and the epoch , we observed additional transits with the TRAPPIST and Euler telescopes .
Using HARPS spectra obtained during the transit , we then determine the projected angle between the spin of the star and the orbit of the planet .

Results : We find that the host star of CoRoT-19b is an inactive F9V-type star close to the end of its main-sequence life .
The host star has a mass M _ { * } = 1.21 \pm 0.05 M _ { \odot } and radius R _ { * } = 1.65 \pm 0.04 R _ { \odot } .
The planet has a mass of M _ { P } = 1.11 \pm 0.06 M _ { Jup } and radius of R _ { P } = 1.29 \pm 0.03 R _ { Jup } .
The resulting bulk density is only \rho = 0.71 \pm 0.06 g cm ^ { -3 } , which is much lower than that for Jupiter .

Conclusions : The exoplanet CoRoT-19b is an example of a giant planet of almost the same mass as Jupiter but a \approx 30 % larger radius .