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Astron. Astrophys. 360, 85-91 (2000)

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5. Conclusion

Using literature data and available OHP frames, we have produced a sample of 26 ellipticals with full colour data in B-V, V-I, B-R and U-B, plus 5 more with only three colours available. The data have been placed in a unique colour system. Some objective estimates of errors have been obtained, but they remain statistically unsufficient.

The gradients in each colour [FORMULA], [FORMULA], [FORMULA] and [FORMULA] have been correlated with the mean [FORMULA] of the first three values. These correlations are good in B-V, V-I, B-R, with [FORMULA] values not larger than estimated errors. It is poor in U-B with a possible contribution of "cosmic scatter". The calculated slopes may be compared with theoretical estimates based upon metallicity variations on the one hand, and upon diffuse dust effects on the other. It appears that the second hypothesis cannot explain the observed relative gradients, while the first one may be more adequate (Table 2).

New observations in U-B are needed to ascertain the respective contributions of real variations and measurement errors in the observed gradients for this colour, as compared to mean gradients such as [FORMULA]. Observed gradients might be due to contributions of both population gradients and diffuse dust in defining the colours.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000