  Context. The pioneer space mission for photometric planet searches, CoRoT,
steadily monitors about 12,000 stars in each of its fields of view; it is able
to detect transit candidates early in the processing of the data and before the
end of a run. Aims. We report the detection of the first planet discovered by
CoRoT and characterizing it with the help of follow-up observations. Methods.
Raw data were filtered from outliers and residuals at the orbital period of the
satellite. The orbital parameters and the radius of the planet were estimated
by best fitting the phase folded light curve with 34 successive transits.
Doppler measurements with the SOPHIE spectrograph permitted us to secure the
detection and to estimate the planet mass. Results. The accuracy of the data is
very high with a dispersion in the 2.17 min binned phase-folded light curve
that does not exceed 3.10-4 in flux unit. The planet orbits a mildly metal-poor
G0V star of magnitude V=13.6 in 1.5 days. The estimated mass and radius of the
star are 0.95+-0.15Msun and 1.11+-0.05Rsun. We find the planet has a radius of
1.49+-0.08Rjup, a mass of 1.03+-0.12Mjup, and a particularly low mean density
of 0.38 +-0.05g cm-3.
