  The emission of prompt fission $\gamma$ rays within a few nanoseconds to a
few microseconds following the scission point is studied in the Hauser-Feshbach
formalism applied to the deexcitation of primary excited fission fragments.
Neutron and $\gamma$-ray evaporations from fully accelerated fission fragments
are calculated in competition at each stage of the decay, and the role of
isomers in the fission products, before $\beta$-decay, is analyzed. The time
evolution of the average total $\gamma$-ray energy, average total $\gamma$-ray
multiplicity, and fragment-specific $\gamma$-ray spectra, is presented in the
case of neutron-induced fission reactions of $^{235}$U and $^{239}$Pu, as well
as spontaneous fission of $^{252}$Cf. The production of specific isomeric
states is calculated and compared to available experimental data. About 7% of
all prompt fission $\gamma$ rays are predicted to be emitted between 10 nsec
and 5 $\mu$sec following fission, in the case of $^{235}$U and $^{239}$Pu
$(n_{\rm th},f)$ reactions, and up to 3% in the case of $^{252}$Cf spontaneous
fission. The cumulative average total $\gamma$-ray energy increases by 2 to 5%
in the same time interval. Finally, those results are shown to be robust
against significant changes in the model input parameters.
