Hard Exclusive Electroproduction of Two Pions off Proton and Deuteron at   HERMES
  Exclusive electroproduction of $\pi^+\pi^-$ pairs off hydrogen and deuterium targets has been studied with the HERMES experiment. The angular distribution of the $\pi^+$ in the $\pi^+\pi^-$ rest system has been studied in the invariant mass range $0.3 < m_{\pi\pi} <1.5$ GeV.   Theoretical models derived in the framework of the Generalized Parton Distributions show that this angular distribution receives only contributions from the interference between the isoscalar channel I=0 and the isovector channel I=1. 
P. di Nezza (1), R. Fabbri (2) (for the HERMES Collaboration) ((1)   INFN-LNF, (2) Ferrara University)