  A single-spin asymmetry in the distribution of exclusively produced pi^+
mesons azimuthally around the virtual photon direction relative to the lepton
scattering plane has been measured for the first time in deep-inelastic
scattering of positrons off longitudinally polarized protons. Integrated over
the experimental acceptance, the sin \phi moment of the polarization asymmetry
of the cross section is measured to be -0.18 +/- 0.05 (stat.) +/- 0.02 (syst.).
The asymmetry is also studied as a function of the relevant kinematic
variables, and its magnitude is found to grow with decreasing x and increasing
-t and vanish at t -> t_min (where x is the Bjorken scaling variable and t is
the squared four-momentum transferred to the nucleon).
