Top: reddened ZAMS fitting to the cluster members with P% > P_min (open circles). The position of the other stars of the sample is indicated by crosses. A full line shows the curve obtained with the fitting algorithm, while the dotted line shows the intrinsic U − B and B − V colours. Bottom: curves representing the limit ΔE(B − V) = 0.11 suggested by Burki (1975). From left to right: Berkeley 86, NGC 2244, NGC 2264, and NGC 6530.