  Single target-spin asymmetries in semi-inclusive two-pion production were
measured for the first time by the HERMES experiment, using a longitudinally
polarized deuterium target. These asymmetries relate to the unknown
transversity distribution function $h_1(x)$ through, also unknown, interference
fragmentation functions. The presented results are compared with a model for
the dependence of one of these interference fragmentation functions on the
invariant mass of the pion pair.
