  We report the first detection of the reactive ions CO+ and HOC+ towards
ultracompact (UC) HII regions, particularly in Mon R2 and G29.96-0.02. We have
observed two positions in Mon R2, namely the peak of the UC HII region and a
position in the high density molecular cloud which bounds it. CO+ and HOC+ were
detected at the UC HII region but not at the molecular cloud, as expected if
the CO$^+$ and HOC$^+$ emissions arise in the PDR surrounding the \uch. The
measured CO$^+$ and HOC$^+$ column densities are of the order of 10$^{11}$
cm$^{-2}$ in both sources, which yields a strikingly low [HCO$^+$]/[HOC$^+$]
abundance ratio of 460 in Mon R2. These values are similar to those found in
some other well-known PDRs, like NGC 7023 and the Orion Bar. We briefly discuss
the chemical implications of these results.
