  The double polarization observable $E$ and the helicity dependent cross
sections $\sigma_{1/2}$ and $\sigma_{3/2}$ were measured for $\eta$
photoproduction from quasi-free protons and neutrons. The circularly polarized
tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used
in combination with a longitudinally polarized deuterated butanol target. The
almost $4\pi$ detector setup of the Crystal Ball and TAPS is ideally suited to
detect the recoil nucleons and the decay photons from $\eta\rightarrow 2\gamma$
and $\eta\rightarrow 3\pi^0$. The results show that the narrow structure
previously observed in $\eta$ photoproduction from the neutron is only apparent
in $\sigma_{1/2}$ and hence, most likely related to a spin-1/2 amplitude.
Nucleon resonances that contribute to this partial wave in $\eta$ production
are only $N1/2^-$ ($S_{11}$) and $N1/2^+$ ($P_{11}$). Furthermore, the
extracted Legendre coefficients of the angular distributions for $\sigma_{1/2}$
are in good agreement with recent reaction model predictions assuming a narrow
resonance in the $P_{11}$ wave as the origin of this structure.
