We present ^ { 12 } CO ( 1-0 ) and ^ { 12 } CO ( 2-1 ) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey , with oxygen abundances ranging from 12 + log ( O/H ) \sim 8.1 to 8.8 .
CO emission is observed in ten galaxies and marginally detected in another one .
CO fluxes correlate with the FIR 250 \mu m emission , and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range .
We compare different methods to estimate H _ { 2 } Â molecular masses , namely a metallicity-dependent CO-to-H _ { 2 } Â conversion factor and one dependent on H -band luminosity .
The molecular-to-stellar mass ratio remains nearly constant at stellar masses \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 10 ^ % { 9 } M _ { \odot } , contrary to the atomic hydrogen fraction , M _ { HI } / M _ { * } , which increases inversely with M _ { * } .
The flattening of the M _ { H _ { 2 } } / M _ { * } ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both H i -deficient and H i -normal dwarfs .
The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity , confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium .
Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals , but gas depletion timescales , \tau _ { dep } , are not constant and range between 100 Myr and 6 Gyr .
The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs , but the molecular gas appears to be less affected at the current stage of evolution within the cluster .
However , the correlation between H i  deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity .