Photometric studies are one of the most valuable and efficient tools for determining the physical properties of open clusters (e.g. distance, age, etc.) and the interstellar matter (ISM) within the cluster as well as along the line of sight of the cluster. The accurate determination of the distance of star clusters is crucial for a wide range of astronomical studies. The interstellar extinction and the ratio of the total-to-selective extinction R = A_V/E(B-V) towards the cluster are important quantities that must be accurately known to determine the distances photometrically.

Observations of star clusters in the near-infrared bands, especially of young clusters which are still embedded in remnants of their parental clouds, are very useful to study the extinction behaviour in these clusters. Observations of interstellar extinction due to the intra-cluster matter yield fundamental information on the optical properties of the particles responsible for extinction. The analysis of extinction curves in 20 OB stellar associations indicates that big complexes are obscured by the same type of interstellar matter (Kiszkurno et al. 1984). On the other hand Kre $\l$ owski & Ströbel (1983) compared the extinction law in two rich stellar aggregates, namely Per OB1 and Sco OB2, situated in different parts of the galactic disk. They found that the average extinction curves of the two aggregates seem to differ substantially. The differences are large in the far-UV, which leads to the conclusion that an average extinction law cannot be applied to all associations. In an analysis of two different complexes of young stars (Sco-Ori region and Perseus region) Kre $\l$ owski & Ströbel (1987) found different shapes of extinction curves, supporting the assumption that the obscuring material in these two young star complexes have different physical properties. Consequently, in the case of young open clusters, the average extinction curve should not be used to correct spectral or photometric data for interstellar extinction.

There is much evidence in the literature that indicates significant variations in the properties of the interstellar extinction and these refer mainly to high values of the total-to-selective extinction ratio R as compared to the normal value, 3.1, for the galactic diffuse medium (see e.g. Pandey et al. 2000 and references therein). Whittet (1977) reported that the value of R in the galactic plane can be represented by a sinusoidal function of the form R = 3.08 + 0.17 sin $(l+175^{\circ})$ , which indicates a minimum value of R at $l \approx 95^{\circ}$ . However, Tapia et al. (1991) found a value of $R=2.42 \pm 0.09$ for the cluster NGC 1502 ( $l=143.7^{\circ}$ ), which is significantly lower than the average galactic value of 3.1.

Table 1: The details of the clusters used in the study.
Cluster l $b\ \ \ \ \$ Distance Log age E(B-V) Available

(pc) (Yr) photometric data

IC 1590 123.13 -06.24 2940 6.54 0.32 $UBVI_{\rm c}JHK$

Be 62 123.99 01.10 2513 7.22 0.85 UBV

NGC 436 126.07 -03.91 2942 7.78 0.48 $UBVI_{\rm c}$

NGC 457 126.56 -04.35 2796 7.15 0.48 $UBVI_{\rm c}$

NGC 581 128.02 -01.76 2241 7.13 0.44 $UBVI_{\rm c}$

Tr 1 128.22 -01.14 2520 7.43 0.63 UBV

NGC 637 128.55 01.70 2372 6.96 0.70 UBV

NGC 654 129.09 -00.35 2422 7.08 0.85 $UBVR_{\rm c}I_{\rm c}$

NGC 663 129.46 -00.94 2284 7.13 0.82 $UBVI_{\rm c}JHK$

Be 7 130.13 00.37 2570 6.60 0.80 UBV

NGC 869 134.63 -03.72 2115 7.10 0.58 $UBVR_{\rm c}I_{\rm c}JHK$

IC 1805 134.74 00.92 2195 6.67 0.80 $UBVR_{\rm c}I_{\rm c}JHK$

NGC 884 135.08 -03.60 2487 7.15 0.58 $UBVR_{\rm c}I_{\rm c}JHK$

NGC 1502 143.65 07.62 900 7.03 0.74 $UBVI_{\rm c}JHK$

The above discussions indicate that the interstellar extinction shows a large range of variability from one line of sight to another. A precise knowledge of the spatial variability of interstellar extinction is important for the following reasons;

**Table 1:** The details of the clusters used in the study.
Cluster	l	$b\ \ \ \ \$	Distance	Log age	E(B-V)	Available
			(pc)	(Yr)		photometric data
IC 1590	123.13	-06.24	2940	6.54	0.32	$UBVI_{\rm c}JHK$
Be 62	123.99	01.10	2513	7.22	0.85	UBV
NGC 436	126.07	-03.91	2942	7.78	0.48	$UBVI_{\rm c}$
NGC 457	126.56	-04.35	2796	7.15	0.48	$UBVI_{\rm c}$
NGC 581	128.02	-01.76	2241	7.13	0.44	$UBVI_{\rm c}$
Tr 1	128.22	-01.14	2520	7.43	0.63	UBV
NGC 637	128.55	01.70	2372	6.96	0.70	UBV
NGC 654	129.09	-00.35	2422	7.08	0.85	$UBVR_{\rm c}I_{\rm c}$
NGC 663	129.46	-00.94	2284	7.13	0.82	$UBVI_{\rm c}JHK$
Be 7	130.13	00.37	2570	6.60	0.80	UBV
NGC 869	134.63	-03.72	2115	7.10	0.58	$UBVR_{\rm c}I_{\rm c}JHK$
IC 1805	134.74	00.92	2195	6.67	0.80	$UBVR_{\rm c}I_{\rm c}JHK$
NGC 884	135.08	-03.60	2487	7.15	0.58	$UBVR_{\rm c}I_{\rm c}JHK$
NGC 1502	143.65	07.62	900	7.03	0.74	$UBVI_{\rm c}JHK$

(i) Since the extinction depends on the optical properties of the dust grains, it can reveal information about the composition and size distribution of the grains. Consequently, variation of the extinction from one direction to other may reveal the degree and nature of dust grain processing in the ISM (Fitzpatrick 1999).

(ii) Since astronomical objects are viewed through interstellar dust, the wavelength dependence of extinction is required to remove the effects of dust obscuration from observed energy distributions.

Uncertainties of extinction estimates limit the accuracy of dereddened energy distributions. Such uncertainties might be acceptably small for very lightly reddened objects but can become important for modestly reddened objects. The intra-cluster extinction due to the remains of the star-forming molecular cloud decreases systematically with the age of the cluster (cf. Pandey et al. 1990).

While analysing the $UBVI_{\rm c}$ data of NGC 663 we found that the V/(U-B)colour-magnitude diagram (CMD) for NGC 663 cannot be explained by a normal value of the extinction ratio E(U-B)/E(B-V)= 0.72 (see Sect. 3). Some clusters in the direction of NGC 663 also appear to show abnormal extinction properties. This motivated us to study the interstellar extinction in detail towards the direction of the cluster NGC 663.

1 Introduction