We calculate the contribution to inclusive high tranverse momentum (pt) charm production at HERA from the excitation of charm in the photon. At large values of pt the results of such a calculation, in the structure function language, will be more reliable as it sums the large logs log (pt2/mc2), as opposed to calculating the contribution of the 2-3 subprocess in fixed order of perturbation theory. We find that this contribution is large and comparable to the contribution from \ fusion production of charm. Suitable cuts on the rapidity of the 'away side' large pt jet allow a very neat separation between the contributions from the excitation process and from pair-production. We further find that including this excitation contribution we can reproduce the measured inclusive D* and \ cross-sections measured by the ZEUS and H1 collaborations respectively, in a LO calculation.
M. et al. (Sun,) studied this question.