**Table 8:** Optical depths estimates from line ratios between isotopologues.
	LkCa15	TW Hya	HD 163296	MWC 480
¹²CO $J\!=\!3\!\rightarrow\!2$	26.8	8.3	18.9	14.9
¹³CO $J\!=\!3\!\rightarrow\!2$	0.44	0.14	0.31	0.25
HCO⁺ $J\!=\!4\!\rightarrow\!3$	19.4	3.8	...	...
H¹³CO⁺ $J\!=\!4\!\rightarrow\!3$	< 0.32	0.06	...	...
HCN $J\!=\!4\!\rightarrow\!3$	0.75	<5.1	...	...
H¹³CN $J\!=\!4\!\rightarrow\!3$	0.012	<0.085	...	...

Note. We assume that the isotopologues have the same excitation
temperature and that [¹²C]/[¹³C]=60.

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