  The gas dissipation from a protoplanetary disk is one of the key processes
affecting planet formation, and it is widely accepted that it happens on
timescales of a few million years for disks around single stars. Over the last
years, several protoplanetary disks have been discovered in multiple star
systems, and despite the complex environment in which they find themselves,
some of them seem to be quite old, a situation that may favor planet formation.
A clear example of this is the disk around HD 98800 B, a binary in a
hierarchical quadruple stellar system, which at a $\sim$10 Myr age seems to
still be holding significant amounts of gas. Here we present a 1D+1D model to
compute the vertical structure and gas evolution of circumbinary disks in
hierarchical triple star systems considering different stellar and disk
parameters.
  We show that tidal torques due to the inner binary together with the
truncation of the disk due to the external companion strongly reduce the
viscous accretion and expansion of the disk. Even allowing viscous accretion by
tidal streams, disks in these kind of environments can survive for more than 10
Myr, depending on their properties, with photoevaporation being the main gas
dissipation mechanism. We particularly apply our model to the circumbinary disk
around HD 98800 B and confirm that its longevity, along with the current
non-existence of a disk around the companion binary HD 98800 A, can be
explained with our model and by this mechanism.
