  Recent results suggest that there is a group of TNOs (2003 EL61 being the
biggest member), with surfaces composed of almost pure water ice and with very
similar orbital elements. We study the surface composition of another TNO that
moves in a similar orbit, 2005 RR43, and compare it with the surface
composition of the other members of this group. We report visible and
near-infrared spectra, obtained with the 4.2m William Herschel Telescope and
the 3.58m Telescopio Nazionale Galileo at the "Roque de los Muchachos"
Observatory (La Palma, Spain). The spectrum of 2005 RR43 is neutral in color in
the visible and dominated by very deep water ice absorption bands in the near
infrared (D= 70.3 +/- 2.1 % and 82.8 +/- 4.9 % at 1.5 \mu and 2.0 \mu
respectively). It is very similar to the spectrum of the group of TNOs already
mentioned. All of them present much deeper water ice absorption bands (D>40 %)
than any other TNO except Charon. Scattering models show that its surface is
covered by water ice, a significant fraction in crytalline state with no trace
(5 % upper limit) of complex organics. Possible scenarios to explain the
existence of this population of TNOs are discussed: a giant collision, an
originally carbon depleted composition, or a common process of continuous
resurfacing. We conclude that TNO 2005 RR43 is member of a group, may be a
population, of TNOs clustered in the space of orbital parameters that show
abundant water ice and no signs of complex organics. The lack of complex
organics in their surfaces suggests a significant smaller fraction of
carbonaceous volatiles like CH4 in this population than in "normal" TNOs. A
carbon depleted population of TNOs could be the origin of the population of
carbon depleted Jupiter family comets already noticed by A'Hearn et al. (1995).
