In order to estimate interstellar extinction to the clusters, we have
plotted apparent (U-B) versus (B-V) diagrams for all the stars
measured by us in the clusters under study (Fig. 8 (click here)). To identify
the main sequence (MS) in these clusters, we have also plotted apparent V,
(B-V); V, (U-B); V, (V-R) and V, (V-I) diagrams for all the
measured stars in Figs. 9 (click here), 10 (click here), 11 (click here) and
12 (click here) for NGC 3228, NGC 4103, NGC 5662 and NGC 6087 respectively.
Adopting the slope of the reddening line E(U-B)/ E(B-V) as 0.72, we have
fitted the intrinsic zero-age main-sequence (ZAMS) given by
Schmidt-Kaler (1982) to the MS stars of spectral type earlier
than A0 in Fig. 8 (click here). This yields a mean value of E(B-V) = 0.31,
0.34 and 0.22 mag with an uncertainty of 
for NGC 4103, NGC 5662 and NGC 6087 respectively. The E(B-V) value
for NGC 3228 could not be estimated because of the lack of MS stars of
spectral type earlier than A0 in our sample. This is evident from the
cluster sequence present in the apparent (U-B) versus (B-V) diagram
(Fig. 8 (click here)a) and the CM diagrams in Fig. 9 (click here). So we adopt the
value of 
for this cluster from the work of
Hogg (1963), based on much brighter stars. The observed
cluster sequences in Fig. 8 (click here) are well-defined for the hotter stars,
indicating that interstellar extinction is uniform across the face of the
observed cluster regions of NGC 4103, NGC 5662 and NGC 6087. Our reddening
estimates for the imaged regions agree fairly well with the values estimated
earlier by others for the same regions (see Sect. 2). In the case of NGC
5662, Clariá et al. (1991) found variable interstellar
extinction across the cluster face which is more than 10 times larger than
the presently imaged area. However, their E(B-V) value for the region
imaged by CCD agrees fairly well with the present estimate.
Figure 8: The (U-B) versus (B-V) diagrams for the stars
observed by us for NGC 3228 (panel a)), NGC 4103 b), NGC 5662
c) and NGC 6087 d). The continuous straight line represents the slope
(0.72) and direction of the reddening vector. The dotted curve
represents the locus of Schmidt-Kaler's (1982) ZAMS, shifted in the
direction of reddening by the values of E(B-V) and E(U-B) indicated in
the diagram
For verifying the nature of the interstellar extinction law in the
direction of each cluster, we used the stars having spectral type
earlier than A0. This has been judged from their location in the (U-B)
versus (B-V) and apparent CM diagrams (Figs. 9 (click here)-12 (click here))
which reveals that bright stars (
) with (B-V) 
in NGC 4103; with (
in NGC 5662 and with (
in NGC 6087, are the desired objects. There are 23, 8 and 10
such stars in NGC 4103, NGC 5662 and NGC 6087 respectively. For these stars,
the 
, E(B-V) and E(U-B) values have been determined using
either the spectral type (available only for 5 stars in NGC 5662 and 1 star
in NGC 6087) taken from the Open Cluster Data Base (cf. Mermilliod
1992), or the UBV photometric Q method (cf. Johnson &
Morgan 1953; Sagar & Joshi 1979) and the calibration
given by Schmidt-Kaler (1982). For calculating E(V-R) and
E(V-I) values, we used the present (V-R) and (V-I) measurements,
Sagar & Cannon's (1994) calibration between 
and
and Walker's (1985) calibration between 
and 
. The mean values of the colour excess ratios derived in this
way are listed in Table 11 (click here). They are in fair agreement with the
normal values, indicating that the law of interstellar extinction in the
direction of the clusters under discussion is normal.
Table 11: A comparison of the colour excess ratios with
E(B-V) for star clusters with the corresponding values for the
normal interstellar extinction law given by Schmidt-Kaler (1982) for
E(U-B)/E(B-V); by Alcalá & Ferro (1988) for E(V-R)/E(B-V) and by
Dean et al. (1978) for E(V-I)/E(B-V)
Figure 9: The V, (B-V); V, (U-B); V, (V-R) and V, (V-I)
diagrams for the stars observed by us in NGC 3228
Figure 10: The V, (B-V); V, (U-B); V, (V-R) and V, (V-I)
diagrams for the stars observed by us in NGC 4103
Figure 11: The V, (B-V); V, (U-B); V, (V-R) and V, (V-I)
diagrams for the stars observed by us in NGC 5662
Figure 12: The V, (B-V); V, (U-B); V, (V-R) and V, (V-I)
diagrams for the stars observed by us in NGC 6087