  We have studied ultrafast dynamics of thin films of LaCoO$_3$ and
La$_{0.5}$Sr$_{0.5}$CoO$_3$ with femtosecond pump-probe ellipsometry in the
energy range of 1.6-3.4 eV. We have observed a large pump-induced transfer of
spectral weight in LaCoO$_3$ that corresponds to an insulator-to-metal
transition. The photo-induced metallic state initially relaxes via a fast
process with a decay constant of about 200 fs. Both LaCoO$_3$ and
La$_{0.5}$Sr$_{0.5}$CoO$_3$ exhibit a significant secondary transient structure
in the 1-30 ps range. Results of measurements on films with different
thicknesses demonstrate that it corresponds to a propagation of an acoustic
strain pulse. On timescales longer than 100 ps, heat diffusion to the substrate
takes place that can be modelled with a bi-exponential decay.
