We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the {it Hubble} Ultra Deep Field (HUDF). Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). Among all the CO emission lines, from cotwoone to CO(6-5), with redshifts accessible via the ASPECS Band~3 and the MUSE data, cotwoone provides the strongest constraints on the molecular gas content. We detect cotwoone emission in galaxies down to stellar masses of $log{(M_*/M_odot)}=10.0$. Below this stellar mass, we present a new constraint on the molecular gas content of $zsim1.5$ main-sequence galaxies by stacking based on the MUSE detections. We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to $log{(M_*/M_odot)}approx9.0$. Assuming a metallicity-based CO--to--$rm H_2$ conversion factor, the molecular gas-to-stellar mass ratio from $log{(M_*/M_odot)}sim9.0$ to $sim10.0$ does not seem to decrease as fast as for $log{(M_*/M_odot)}>10.0$, which is in line with simulations and studies at lower redshift. The inferred molecular gas density $rho{rm (H_2)}=(0.49pm0.09)times10^8,{rm M_odot,Mpc^{-3}}$ of MUSE-selected galaxies at $zsim1.5$ is comparable with the one derived in the HUDF with a different CO selection. Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wavebands.
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