The Centaurs are a particular family of objects with orbits whose semimajor axes fall between those of Jupiter and Neptune. They are likely the transition objects between the Kuiper belt population and short-period comets. To investigate the nature of these particular objects, we have performed optical spectroscopic observations of five Centaurs. The results show a great diversity among the reflectances of the five Centaurs. The colors do not seem to be related to the perihelion distance of the objects. We looked for weak cometary emission features, in particular the CN-band emission at 3880 ?, but no CN emission feature has been detected within 3 σ in any of the investigated spectra.
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