Among the 160 known exoplanets , mainly detected in large radial-velocity surveys , only 8 have a characterization of their actual mass and radius thanks to the two complementary methods of detection : radial velocities and photometric transit .

We started in March 2004 an exoplanet-search programme biased toward high-metallicity stars which are more frequently host extra-solar planets .
This survey aims to detect close-in giant planets , which are most likely to transit their host star .

For this programme , high-precision radial velocities are measured with the ELODIE fiber-fed spectrograph on the 1.93-m telescope , and high-precision photometry is obtained with the CCD Camera on the 1.20-m telescope , both at the Haute-Provence Observatory .

We report here the discovery of a new transiting hot Jupiter orbiting the star HD 189733 .
The planetary nature of this object is confirmed by the observation of both the spectroscopic and photometric transits .
The exoplanet HD 189733 b , with an orbital period of 2.219 days , has one of the shortest orbital periods detected by radial velocities , and presents the largest photometric depth in the light curve ( \sim 3 % ) observed to date .
We estimate for the planet a mass of 1.15 \pm 0.04 M _ { J } and a radius of 1.26 \pm 0.03 R _ { J } .
Considering that HD 189733 has the same visual magnitude as the well known exoplanet host star HD 209458 , further ground-based and space-based follow-up observations are very promising and will permit a characterization of the atmosphere and exosphere of this giant exoplanet .

Keywords . stars : individual : HD 189733 – planetary systems – techniques : radial velocities – techniques : photometry