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4 Discussion and conclusions

In this work we confirm the previous classification of several asteroids, as well as attribute a taxonomic class, according to the more recent taxonomic system developed, to some other objects never classified before. Fifty-seven spectra have been obtained for Mars-crossing asteroids and twenty-seven for Near-Earth asteroids in the visible wavelength. In this region only part of the absorption features due to olivine and/or pyroxene are in evidence. Therefore, to better characterize the mineralogy of the observed objects we would need observations in the near-infrared region.

Differences between our spectra and the SMASS ones, when observed, are small, with only one exception, the Mars-crosser 1139 Atami. The disagreements in the classifications are, in most cases, due to differences in the slope (or redness) of the spectra, which can be due to atmospheric and/or instrumental conditions. On the other hand, several NEAs of our sample have been observed at large solar phase angles (see Table 2), which could lead to a "phase reddening effect'', as discussed by Luu & Jewitt (1990). However, due to the small temporal coverage of our observations it is not possible to precisely quantify this reddening, if it exists at all. Future work is needed in order to clarify this effect and the way in which it operates on asteroids.

No relations were found between reflectivity gradients and asteroid diameters in the studied set of asteroids. In spite of the small sample, with the spectra presented in this work we give an additional example of the variety of taxonomic classes and, probably, mineralogies, present in the Mars-crossing and NEA populations. Through the comparison of the Mars-crossing and NEA spectra versus OC spectra we show, once more, that objects resembling this class of meteorites are present in both these populations. Additional studies are needed to better quantify these populations and their relationships.

Acknowledgements
We acknowledge Edward F. Tedesco for comments and suggestions that much improved the paper, and the technical staff of ESO for their prompt help whenever needed. Financial support by CNPq and FAPERJ during the present research is also acknowledged.


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