6 Conclusions

The present observations of colour distributions in 12 S0 galaxies, most of these nearly edge-on, suggest the following conclusions:

1.

Radial isophotal colour distributions show some outwards blueing as in E type galaxies. The relative gradients in the 4 colour indices B-V, V-I, B-R, U-V, are not proportional to relative dust reddening coefficients, the V-I gradient beeing quite small and the U-V one quite large: apparently dust cannot be responsible for such gradients. When it is feasible to measure both the bulge and disk colour gradients the first one is steeper;

2.

Optical dust is detected in 10 of the studied objects out of 12, either from definite dust patterns (NGC 2549, 3245, 4026, 4036, 4111, 5422, 7332), or from indirect but ambiguous evidence. It is observed that the "peanut bulge" of NGC 4111 possibly results from the central dust pattern in this object;

3.

Dust is likely to be present, without forming clear patterns, in the disk of NGC 4350, 5308, and perhaps also in the inner "decoupled disk" of NGC 3115. In these 3 cases the evidence comes from reddening of the majA as compared to its surroundings, along isophotal contours: it remains ambiguous however, since population differences might give similar observational effects. Checks from the use of various colour indices are not as yet conclusive;

4.

A disk slightly bluer than the adjoining regions is found in NGC 3098 and NGC 3115, but only at $a > 15\hbox{$^{\prime\prime}$ }$ or so in this later case;

5.

For a number of objects, the minA regions along a given isophote, become bluer than the majA tips. This might perhaps be explained in terms of spheroid + disk models, assuming a a strong outwards blue gradient in the spheroid, against a flater colour disk. By "spheroid" we here mean the superposition of the bulge itself and a thD or spH envelope, to use the classification scheme of MM93, much less flat than the disk. In principle some dust concentration in the disk might also provide an explanation, but our evidence from the comparison of the B-V and V-I colour indices does not support it;

6.

Some correlations are found between the ring struture and the colour along the majA in NGC 2549 and 4026, more pronounced in U-V: there a blueing of the colour profile coincides with the upwards "humps" in the SuBr profiles.