Transiting exoplanets from the CoRoT space mission: VII. The
  "hot-Jupiter"-type planet CoRoT-5b
  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
anticenter direction. CoRoT-5b is a "hot Jupiter-type" planet with a radius of
1.388(+0.046, -0.047) R_Jup, a mass of 0.467(+0.047, -0.024) M_Jup, and
therefore, a mean density of 0.217(+0.031, -0.025) 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(+0.00026,
  -0.00029) AU.
