Evolution of ΔY vs. ΔO for our two best models, M4C (solid line) and M1R (dashed line). The black square represents observed abundances under the assumption of constant temperature, and the open square represents abundances from RLs under the assumption of temperature variations (Peimbert et al. 2005), note that the observed ΔY values might be lower limits due to the possible presence of neutral helium inside the H ii region.