  We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations
towards 27 low-redshift ($0.02< z<0.2$) star-forming galaxies taken from the
Valpara\'iso ALMA/APEX Line Emission Survey (VALES). We perform stacking
analyses of the $^{12}$CO($1-0$), $^{13}$CO($1-0$) and C$^{18}$O($1-0$)
emission lines to explore the $L'$ ($^{12}$CO($1-0$))/$L'$($^{13}$CO($1-0$)))
(hereafter $L'$($^{12}$CO)/$L'$($^{13}$CO)) and
$L'$($^{13}$CO($1-0$))/$L'$(C$^{18}$O($1-0$)) (hereafter
$L'$($^{13}$CO)/$L'$(C$^{18}$O) line luminosity ratio dependence as a function
of different global galaxy parameters related to the star formation activity.
The sample has far-IR luminosities $10^{10.1-11.9}$L$_{\odot}$ and stellar
masses of $10^{9.8-10.9}$M$_{\odot}$ corresponding to typical star-forming and
starburst galaxies at these redshifts. On average we find a
$L'$($^{12}$CO)/$L'$($^{13}$CO) line luminosity ratio value of 16.1$\pm$2.5.
Galaxies with evidences of possible merging activity tend to show higher
$L'$($^{12}$CO)/$L'$($^{13}$CO) ratios by a factor of two, while variations of
this order are also found in galaxy samples with higher star formation rates or
star formation efficiencies. We also find an average
$L'$($^{13}$CO)/$L'$(C$^{18}$O) line luminosity ratio of 2.5$\pm$0.6, which is
in good agreement with those previously reported for starburst galaxies. We
find that galaxy samples with high $L_{\text{IR}}$, SFR and SFE show low
$L'$($^{13}$CO)/$L'$(C$^{18}$O) line luminosity ratios with high
$L'$($^{12}$CO)/$L'$($^{13}$CO) line luminosity ratios, suggesting that these
trends are produced by selective enrichment of massive stars in young
starbursts.
