  We present the discovery of a candidate multiply-transiting system, the first
one found in the CoRoT mission. Two transit-like features with periods of 5.11
and 11.76d are detected in the CoRoT light curve, around a main sequence K1V
star of r=15.1. If the features are due to transiting planets around the same
star, these would correspond to objects of 3.7$\pm$0.4 and 5.0$\pm$0.5 R_earth
respectively. Several radial velocities serve to provide an upper limit of 5.7
M_earth for the 5.11~d signal, and to tentatively measure a mass of
28$^{+11}_{-11}$ M_earth for the object transiting with a 11.76~d period. These
measurements imply low density objects, with a significant gaseous envelope.
The detailed analysis of the photometric and spectroscopic data serve to
estimate the probability that the observations are caused by transiting
Neptune-sized planets as $>$26$\times$ higher than a blend scenario involving
only one transiting planet, and $>$900$\times$ higher than a scenario involving
two blends and no planets. The radial velocities show a long term modulation
that might be attributed to a 1.5 M_jup planet orbiting at 1.8~A.U. from the
host, but more data are required to determine the precise orbital parameters of
this companion.
