The extinction law and the variation of colour excess with position, luminosity and spectral class in young open star clusters NGC 663, NGC 869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14, Tr 15, Tr 16, Coll 228, Tr 37 and Be 86 have been studied. The difference in the minimum and maximum values of of cluster members has been considered as a measure of the presence of non-uniform gas and dust inside the clusters. Its value ranges from 0.22 to 1.03 mag in the clusters under study, which indicates that non-uniform extinction is present in all the clusters. It has been noticed for the first time in NGC 1502 and Tr 37. It is also found that the differential colour excess in open clusters, which may be caused by the presence of gas and dust, decreases systematically with the age of clusters, indicating that matter is either used in star formation or blown away by hot stars or both. There is no uniformity in the variation of with position or spectral class or luminosity. Except in Tr 14, all clusters show a random spatial distribution of , indicating a random distribution of gas and dust inside the clusters. The value correlates with both luminosity and spectral class only in the case of Coll 228, Tr 16 and Be 86. The members of these clusters at show larger values of colour excess ratios than the normal ones. The value of for most of the cluster members is close to the normal interstellar value of 1.73. However, the colour excess ratios with ) at are smaller than the normal value for NGC 663, NGC 869, NGC 884 and NGC 1502, while they are larger for NGC 6611, Coll 228, Tr 16 and Tr 14. Thus there is no uniformity in the relationship of extinction properties amongst the clusters under study.

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