We report the detection of fully resolved absorption lines of A-X bands from interstellar 12C17O and 12C18O, through high-resolution spectroscopy of X Per with the Space Telescope Imaging Spectrograph. The first ultraviolet measurement of an interstellar 12C17O column density shows that its isotopomeric ratio is 12C16O/12C17O = 8700 pm 3600. Simultaneously, the second ultraviolet detection of interstellar 12C18O establishes its isotopomeric ratio at 3000 pm 600. These ratios are about five times higher than local ambient oxygen isotopic ratios in the ISM. Such severe fractionation of rare species shows that both 12C17O and 12C18O are destroyed by photodissociation, whereas 12C16O avoids destruction through self-shielding. This is to be contrasted with our ratio of 12C16O/13C16O = 73 pm 12 toward X Per, which is indistinguishable from 12C/13C, the result of a balance between photodissociation of 13C16O and its preferential formation via the isotope exchange reaction between CO and C^+.
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