  Four young star clusters were studied in order to characterize their
anomalous extinction or variable reddening that could be due to a possible
contamination by dense clouds or circumstellar effects. The extinction law (Rv)
was evaluated by adopting two methods: (i) the use of theoretical expressions
based on the colour-excess of stars with known spectral type, and (ii) the
analysis of two-colour diagrams, where the slope of observed colours
distribution is compared to the normal distribution. An algorithm to reproduce
the zero age main sequence (ZAMS) reddened colours was developed in order to
derive the average visual extinction (Av) that provides the best fitting of the
observational data. The structure of the clouds was evaluated by means of
statistical fractal analysis, aiming to compare their geometric structure with
the spatial distribution of the cluster members. The cluster NGC 6530 is the
only object of our sample showing anomalous extinction. In average, the other
clusters are suffering normal extinction, but several of their members, mainly
in NGC 2264, seem to have high Rv, probably due to circumstellar effects. The
ZAMS fitting provides Av values that are in good agreement with those found in
the literature. The fractal analysis shows that NGC 6530 has a centrally
concentrated distribution of stars that is different of the sub-structures
found in the density distribution of the cloud projected in the Av map,
suggesting that the original cloud has been changed with the cluster formation.
On the other hand, the fractal dimension and the statistical parameters of
Berkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation,
when compared to other works based on artificial distribution of points.
