  This paper is intended as part of a more extensive molecular line survey in
star forming regions along the evolutionary track of a collapsing cloud toward
a young stellar object. We have studied a sample of seven small dark clouds
(Bok globules) and eight Herbig Ae/Be stars in the J=1->0 transition of
HCO$^{+}$, H$^{13}$CO$^{+}$, HCN and H$^{13}$CN. The choice of these molecules
is determined by the simple chemistry and the predicted high abundance of the
reactants leading to their formation. The isotopically substituted species
(isotopomers), H$^{13}$CO$^{+}$ and H$^{13}$CN, were observed in order to
determine, whenever possible, the optical thickness of the main species. The
most abundant isotopomers were found in almost all the sources (detection rate
70-90\%). Those sources which exhibited the strongest signals were also
searched for the $^{13}$C isotopomers. H$^{13}$CO$^{+}$ was found in one dark
cloud and around three Herbig Ae/Be stars, while H$^{13}$CN around only one
star. The column densities for each species and the physical conditions of the
objects were derived whenever the observational data allowed it.
