  Single-spin asymmetries for pions and charged kaons are measured in
semi-inclusive deep-inelastic scattering of positrons and electrons off a
transversely nuclear-polarized hydrogen target. The dependence of the cross
section on the azimuthal angles of the target polarization (phi_S)and the
produced hadron (phi) is found to have a substantial sin(phi+phi_S) modulation
for the production of pi+, pi- and K+. This Fourier component can be
interpreted in terms of non-zero transversity distribution functions and
non-zero favored and disfavored Collins fragmentation functions with opposite
sign. For pi0 and K- production the amplitude of this Fourier component is
consistent with zero.
