  Determining molecular abundance ratios is important not only for the study of
the Galactic chemistry but also because they are useful to estimate physical
parameters in a large variety of interstellar medium environments. The CO is
one of the most important molecules to trace the molecular gas in the
interstellar medium, and the 13CO/C18O abundance ratio is usually used to
estimate molecular masses and densities of regions with moderate to high
densities. Nowadays this kind of isotopes ratios are in general indirectly
derived from elemental abundances ratios. We present the first 13CO/C18O
abundance ratio study performed from CO isotopes observations towards a large
sample of Galactic sources of different nature at different locations. To study
the 13CO/C18O abundance ratio it was used 12CO J=3-2 data obtained form the CO
High-Resolution Survey, 13CO and C18O J=3-2 data from the 13CO/C18O (J=3-2)
Heterodyne Inner Milky Way Plane Survey, and some complementary data extracted
from the James Clerk Maxwell Telescope database. It was analyzed a sample of
198 sources composed by young stellar objects (YSOs), HII and diffuse HII
regions as catalogued in the Red MSX Source Survey in 27.5 < l < 46.5 and |b| <
0.5. Most of the analyzed sources are located in the galactocentric distance
range 4.0-6.5 kpc. We found that YSOs have, in average, smaller 13CO/C18O
abundance ratios than HII and diffuse HII regions. Taking into account that the
gas associated with YSOs should be less affected by the radiation than in the
case of the others sources, selective far-UV photodissociation of C18O is
confirmed. The 13CO/C18O abundance ratios obtained in this work are
systematically lower than the predicted from the known elemental abundance
relations. These results would be useful in future studies of molecular gas
related to YSOs and HII regions based on the observation of these isotopes.
