6 Conclusions

In the present work we have carried out a detailed study of the intra-cluster material inside the young open clusters within a range of galactic longitude $123^{\circ} <l< 144^{\circ}$. We used three methods (cf. Sect. 3.1) to derive $E(\lambda -V)/E(B-V)$ and the results agree with each other in a few percent. It is found that the behaviour of extinction varies from cluster to cluster. The main results are as follows.

1.

The extinction curves at shorter wavelengths depend upon the $R_{\rm cluster}$ while they converge for $\lambda \geq \lambda_{J}$.

2.

The extinction behaviour in the case of NGC 654, NGC 869 and NGC 884 is found normal, whereas in the case of NGC 663 there is some tendency for a higher value of $R_{\rm cluster}$. It is interesting to mention that in the case of NGC 663 and NGC 869 the extinction at $\lambda_U$ is found different from the normal one. In the case of NGC 663 the extinction process in the U band seems to be less efficient, whereas in the case of NGC 869 the process is more efficient.

3.

The cluster regions of IC 1502 and IC 1805 show anomalous reddening laws. In the case of IC 1502 the $R_{\rm cluster} \sim 2.57\pm0.16$ which is in agreement with the value ($\sim$ $2.49\pm0.09$) obtained by Tapia et al. (1991), whereas in the case of IC 1805 the present study yields $R_{\rm cluster}\sim3.53\pm0.25$ which is in agreement with the value ($\sim$3.44) given by Kwon & Lee (1983).

More near-IR data is needed for the clusters NGC 663, NGC 1502 and IC 1805, where an anomalous value of $R_{\rm cluster}$ is found, to study the reddening law in detail in the these clusters.

Acknowledgements

This work is partly supported by the DST (India) and the JSPS (Japan). AKP is thankful to the staff of KISO observatory for their help during his stay there. Authors are grateful to Prof. Ram Sagar for useful discussions. Thanks are also due to Dr. John Mathis for his comments which improved the contents of the paper.