¹⁶O/¹⁸O ratios after the first dredge-up versus stellar mass in M_⊙. Lines represent the theoretical predictions, and the red squares are observations. Solid blue line: reference model (R). Dashed blue lines: ¹⁸O proton capture rate modified within the suggested upper and lower rates (O18L and O18H). Solid black line: model predictions obtained by reducing the initial ¹⁶O to [¹⁶O/H] = –0.22 (C16OL). Dashed black line: model with the initial ¹⁸O abundance increased by a factor of 2 (C18OHH).