Observational constraints on disc sizes in protoplanetary discs in
  multiple systems in the Taurus region. II. Gas disc sizes
  The formation of multiple stellar systems is a natural by-product of the
star-formation process, and its impact on the properties of protoplanetary
discs and on the formation of planets is still to be fully understood. To date,
no detailed uniform study of the gas emission from a sample of protoplanetary
discs around multiple stellar systems has been performed. Here we analyse new
ALMA observations at a $\sim$21 au resolution of the molecular CO gas emission
targeting discs in eight multiple stellar systems in the Taurus star-forming
regions. $^{12}$CO gas emission is detected around all primaries and in seven
companions. With these data, we estimate the inclination and the position angle
for all primary discs and for five secondary or tertiary discs, and measure the
gas disc radii of these objects with a cumulative flux technique on the
spatially resolved zeroth moment images. When considering the radius including
95\% of the flux as a metric, the estimated gas disc size in multiple stellar
systems is found to be on average $\sim 4.2$ times larger than the dust disc
size. This ratio is higher than what was recently found in a population of more
isolated and single systems. On the contrary, when considering the radius
including 68\% of the flux, no difference between multiple and single discs is
found in the distribution of ratios. This discrepancy is due to the sharp
truncation of the outer dusty disc observed in multiple stellar systems. The
measured gas disc sizes are consistent with tidal truncation models in multiple
stellar systems assuming eccentricities of $\sim0.15$-$0.5$, as expected in
typical binary systems.
