Oxygen isotope fractionation of protostellar systems relative to the ISM (black circle). Solid and dashed lines have a slope of 0.5 and 1, as expected respectively for mass-independent and mass-dependent isotopic fractionation. XUE 10 is shown with a star marker and error bars, and the derived ratios are also labeled with uncertainties. The green and orange square markers refer to the values derived from CO₂ emission toward MY Lup with different assumptions (Salyk et al. 2025). The blue circles indicate the isotopic ratios derived from CO emission toward the sample of nearby young solar analogs (YSOs) from Smith et al. (2015).