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We present a model of generalised parton distributions based on a forwardansatz in the DGLAP region. We discuss some aspects of the parametrisations, asthe dependence in t, with factorised and non-factorised approaches, where t isthe square of the four-momentum exchanged at the hadron vertex. The predictionsof this model are then compared with DVCS cross sections from H1 and ZEUS, anda related observable, the skewing factor, defined as the following ratioimaginary amplitudes : $R \equiv {Im {\cal A} (\gamma^*+p \to \gamma+p)\lfrestriction{t=0}} / {Im {\cal A} (\gamma^*+p \to \gamma^*+p)\lfrestriction{t=0}}$. It is an interesting quantity including both thenon-forward kinematics and the non-diagonal effects. Finally, we discuss thebeam charge asymmetry, which is certainly the most sensitive observable to thedifferent hypothesis needed in the GPDs parametrisations. We show that theapproximations done for the t dependence lead to significant differences forthe predictions in the HERMES kinematic domain and prospects are given forCOMPASS.

Generalised parton distributions at HERA and prospects for COMPASS