Timing of secondary maximum (t₂(x), top) and normalised integrated flux in the range of 15–40 d (ℱ_x2, bottom) in the r band (left) and i band (right) as a function of Δm₁₅(g) for our sample. We also show simple linear regression fits to the whole sample (black dashed lines). We find a non-zero slope at the > 5σ confidence level for t₂(r), t₂(i), ℱ_r2, but not for ℱ_i2 with Δm₁₅(g).