  F\"orster theory describes electronic exciton energy migration in molecular
assemblies as an incoherent hopping process between donor and acceptor
molecules. The rate is expressed in terms of the overlap integral between donor
fluorescence and acceptor absorption spectra. Typical time scales for systems
like photosynthetic antennae are on the order of a few picoseconds. Prior to
transfer it is assumed that the initially excited donor molecule has
equilibrated with respect to the local environment. However, upon excitation
and during the equilibration phase the state of the system needs to be
described by the full density matrix, including coherences between donor and
acceptor states. While being intuitively clear, addressing this regime
experimentally has been a challenge until the recently reported advances in
Fluorescence Detected Two-Dimensional Spectroscopy (FD2DS). Here, we
demonstrate using fourth order perturbation theory, the conditions for the
presence of donor-acceptor coherence induced cross-peaks at zero waiting time
between the first and the second pair of pulses. The approach is illustrated
for a heterodimer model which facilitates an analytical solution.
