Abstract

Aims. The CoRoT space mission continues to photometrically monitor about 12 000 stars in its field-of-view for a series of target fields to search for transiting extrasolar planets ever since 2007. Deep transit signals can be detected quickly in the "alarm-mode" in parallel to the ongoing target field monitoring. CoRoT's first planets have been detected in this mode. Methods. The CoRoT raw lightcurves are filtered for orbital residuals, outliers, and low-frequency stellar signals. The phase folded lightcurve is used to fit the transit signal and derive the main planetary parameters. Radial velocity follow-up observations were initiated to secure the detection and to derive the planet mass. Results. We report the detection of CoRoT-5b, detected during observations of the LRa01 field, the first long-duration field in the galactic anti-center direction. CoRoT-5b is a "hot Jupiter-type" planet with a radius of 1.388(-0.047)(+0.046) R(Jup), amass of 0.467(-0.024)(+0.047) M(Jup), and therefore, a mean density of 0.217(-0.025)(+0.031) g cm(-3). The planet orbits an F9V star of 14.0 mag in 4.0378962 +/- 0.0000019 days at an orbital distance of 0.04947(-0.00029)(+0.00026) AU.