  We report the detection of CoRoT-23b, a hot Jupiter transiting in front of
its host star with a period of 3.6314 \pm 0.0001 days. This planet was
discovered thanks to photometric data secured with the CoRoT satellite,
combined with spectroscopic radial velocity (RV) measurements. A photometric
search for possible background eclipsing binaries conducted at CFHT and OGS
concluded with a very low risk of false positives. The usual techniques of
combining RV and transit data simultaneously were used to derive stellar and
planetary parameters. The planet has a mass of Mp = 2.8 \pm 0.3 MJup, a radius
of Rpl = 1.05 \pm 0.13 RJup, a density of \approx 3 g cm-3. RV data also
clearly reveal a non zero eccentricity of e = 0.16 \pm 0.02. The planet orbits
a mature G0 main sequence star of V =15.5 mag, with a mass M\star = 1.14 \pm
0.08 M\odot, a radius R\star = 1. 61 \pm 0.18 R\odot and quasi-solar
abundances. The age of the system is evaluated to be 7 Gyr, not far from the
transition to subgiant, in agreement with the rather large stellar radius. The
two features of a significant eccentricity of the orbit and of a fairly high
density are fairly uncommon for a hot Jupiter. The high density is, however,
consistent with a model of contraction of a planet at this mass, given the age
of the system. On the other hand, at such an age, circularization is expected
to be completed. In fact, we show that for this planetary mass and orbital
distance, any initial eccentricity should not totally vanish after 7 Gyr, as
long as the tidal quality factor Qp is more than a few 105, a value that is the
lower bound of the usually expected range. Even if Corot-23b features a density
and an eccentricity that are atypical of a hot Jupiter, it is thus not an
enigmatic object.
