Relation between the mass values (M_⊙) derived respectively from ¹³CO J = 1−0 and ¹²CO J = 2−1 30 m telescope data for the objects in which both determinations can be performed (Sect. 5). Results from ¹³CO J = 1−0 are expected to be the most accurate ones and those from ¹²CO J = 2−1 are underestimates. The continuous line represents a factor of ten between both estimates, and the other lines represent deviations of a 10% from this ratio.