  We report a first measurement of the double-polarisation observable,
$C_{x'}$, in $\pi^+$ photoproduction off the proton. The $C_{x'}$
double-polarisation observable represents the transfer of polarisation from a
circularly polarised photon beam to the recoiling neutron. The MAMI circularly
polarised photon beam impinged on a liquid deuterium target cell, with reaction
products detected in the Crystal Ball calorimeter. Ancillary apparatus
surrounding the target provided tracking, particle identification and
determination of recoil nucleon polarisation. The $C_{x'}$ observable is
determined for photon energies 800-1400 MeV, providing new constraints on
models aiming to elucidate the spectrum and properties of nucleon resonances.
This is the first determination of any polarisation observable from the
beam-recoil group of observables for this reaction, providing a valuable
constraint and systematic check of the current solutions of partial wave
analysis based theoretical models.
