  We study the visible and near-infrared (NIR) spectral properties of different
ACO populations and compare them to the independently determined properties of
comets.
  We select our ACOs sample based on published dynamical criteria and present
our own observational results obtained using the 10.4m Gran Telescopio Canarias
(GTC), the 4.2m William Herschel Telescope (WHT), the 3.56m Telescopio
Nazionale Galileo (TNG), and the 2.5m Isaac Newton Telescope (INT), all located
at the El Roque de los Muchachos Observatory (La Palma, Spain), and the 3.0m
NASA Infrared Telescope Facility (IRTF), located at the Mauna Kea Observatory,
in Hawaii. We include in the analysis the spectra of ACOs obtained from the
literature. We derive the spectral class and the visible and NIR spectral
slopes. We also study the presence of hydrated minerals by studying the 0.7
$\mu$m band and the UV-drop below 0.5 $\mu$m associated with phyllosilicates.
  We present new observations of 17 ACOs, 11 of them observed in the visible, 2
in the NIR and 4 in the visible and NIR. We also discuss the spectra of 12 ACOs
obtained from the literature.
  All but two ACOs have a primitive-like class spectrum (X or D-type). Almost
100\% of the ACOs in long-period cometary orbits (Damocloids) are D-types.
Those in Jupiter family comet orbits (JFC-ACOs) are $\sim$ 60\% D-types and
$\sim$ 40\% X-types. The mean spectral slope $S'$ of JFC-ACOs is 9.7 $\pm$ 4.6
\%/1000 \AA \ and for the Damocloids this is 12.2 $\pm$ 2.0 \%/1000 \AA . No
evidence of hydration on the surface of ACOs is found from their visible
spectra. The slope and spectral class distribution of ACOs is similar to that
of comets.
  The spectral classification, the spectral slope distribution of ACOs, and the
lack of spectral features indicative of the presence of hydrated minerals on
their surface, strongly suggest that ACOs are likely dormant or extinct comets.
