  We have measured the deuterium fractionation (through the column density
ratio N(N2D+)/N(N2H+)) and the CO depletion factor (ratio between expected and
observed CO abundance) in a sample of 10 high-mass protostellar candidates, in
order to understand whether the earliest evolutionary stages of high-mass stars
have chemical characteristics similar to those of low-mass ones. The
observations were carried out with the IRAM-30m telescope and the JCMT. We have
detected N2D+ emission in 7 of the 10 sources of our sample, and found an
average value N(N2D+)/N(N2H+)~0.015. This value is 3 orders of magnitude larger
than the interstellar D/H ratio, indicating the presence of cold and dense gas,
in which the physical-chemical conditions are similar to those observed in
low-mass pre-stellar cores. Also, the integrated CO depletion factors show that
in the majority of the sources the expected CO abundances are larger than the
observed values, with a median ratio of 3.2. In principle, the cold gas that
gives origin to the N2D+ emission can be the remnant of the massive molecular
core in which the high-mass (proto-)star was born, not yet heated up by the
central object. If so, our results indicate that the chemical properties of the
clouds in which high-mass stars are born are similar to their low-mass
counterparts. Alternatively, this cold gas can be located into one (or more)
starless core (cores) near the protostellar object. Due to the poor angular
resolution of our data, we cannot decide which is the correct scenario.
