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