DESY-HERA. Measurement of double diffractive production in gamma p interactions in events with a large rapidity gap in between the photon and proton diffractive systems. Data are given as the differential cross section as a function of the invariant mass of the system produced at the photon vertex at two centre of mass energies, 187 and 231 GeV, for both the condition where the proton remains intact and when it dissociates. The data where taken in 1996 in two runs, first with vertex detector in its normal position and second with it shifted. The corresponding luminosities are 24.7 +- 0.4 and 23.8 +- 1.3 nb-1 respectively.