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Using very high resolution (lambda / Dlambda ~ 95000) 4.7 micron fundamentaland 2.3 micron overtone ro-vibrational CO absorption spectra obtained with theCRIRES infrared spectrometer on the Very Large Telescope (VLT), we reportdetections of four CO isotopologues -- C16O, 13CO, C18O and the rare species,C17O -- in the circumstellar environment of two young protostars, VV CrA andReipurth 50. We argue that the observed CO absorption lines probe aprotoplanetary disk in VV CrA, and a protostellar envelope in Reipurth 50. AllCO line profiles are spectrally resolved, permitting direct calculation of COoxygen isotopologue ratios with 5-10% accuracy. The ro-vibrational levelpopulations for all species can be reproduced by assuming that CO absorptionarises in two temperature regimes. For both objects, 12C/13C are on the orderof 100, nearly twice the expected interstellar medium (ISM) ratio. The derivedoxygen abundance ratios for the VV CrA disk show a significant mass-independentdeficit of C17O and C18O relative to C16O compared to ISM baseline abundances.The Reipurth 50 envelope shows no clear differences in oxygen CO isotopologueratios compared with the local ISM. A mass-independent fractionation can beinterpreted as being due to selective photodissociation of CO in the disksurface due to self-shielding. The deficits in C17O$ and C18O$ in the VV CrAprotoplanetary disk are consistent with an analogous origin of the 16Ovariability in the solar system by isotope selective photodissociation,confirmation of which may be obtained via study of additional sources. Thehigher fractionation observed for the VV CrA disk compared with the Reipurth 50envelope is likely due to a combination of disk geometry, grain growth, andvertical mixing processes. [Abstract abridged]

High-precision C17O, C18O and C16O measurements in young stellar objects: analogues for CO self-shielding in the early solar system