  We present rotation period measurements for subgiants observed by CoRoT.
Interpreting the modulation of stellar light that is caused by star-spots on
the time scale of the rotational period depends on knowing the fundamental
stellar parameters. Constraints on the angular momentum distribution can be
extracted from the true stellar rotational period. By using models with an
internal angular momentum distribution and comparing these with measurements of
rotation periods of subgiant stars we investigate the agreement between
theoretical predictions and observational results. With this comparison we can
also reduce the global stellar parameter space compatible with the rotational
period measurements from subgiant light curves. We can prove that an evolution
assuming solid body rotation is incompatible with the direct measurement of the
rotational periods of subgiant stars. Measuring the rotation periods relies on
two different periodogram procedures, the Lomb-Scargle algorithm and the
Plavchan periodogram. Angular momentum evolution models were computed to give
us the expected rotation periods for subgiants, which we compared with measured
rotational periods. We find evidence of a sinusoidal signal that is compatible
in terms of both phase and amplitude with rotational modulation. Rotation
periods were directly measured from light curves for 30 subgiant stars and
indicate a range of 30 to 100 d for their rotational periods. Our models
reproduce the rotational periods obtained from CoRoT light curves. These new
measurements of rotation periods and stellar models probe the non-rigid
rotation of subgiant stars.
