  We performed femtosecond reflection spectroscopy on a series of
perovskite-type cobalt oxide $R$BaCo$_2$O$_{6-\delta}$ ($R$=Sm, Gd, and Tb)
crystals, in which the electronic transfer was controlled by $R$.
  The transient reflectivity and the optical conductivity ($\sigma^{\rm
PI}(\omega)$) obtained by Kramers-Kronig analysis showed an ultrafast change
within a time resolution ($\approx 150$ fs) at room temperature and the
appearance of signals of a hidden state different from the high temperature
metallic state. The transferred spectral weight in $\sigma^{\rm PI}(\omega)$
upon photoexcitation sensitively depended on the $R$-species, indicating an
important role of electronic correlation in the photoexcited state.
