  We present models of spherically symmetric recurrent nova shells interacting
with circumstellar material in a symbiotic system composed of a red giant
expelling a wind, and a white dwarf accreting from this material. Recurrent
nova eruptions periodically eject material at high velocities ($\gtrsim 10^3$
km/s) into the red giant wind profile, creating a decelerating shock wave as
circumstellar material is swept up. High circumstellar material densities cause
the shocked wind and ejecta to have very short cooling times of days to weeks.
Thus, the late time evolution of the shell is determined by momentum
conservation instead of energy conservation. We compute and show evolutionary
tracks of shell deceleration, as well as post-shock structure. After sweeping
up all the red giant wind, the shell coasts at a velocity $\sim 100$ km/s,
depending on system parameters. These velocities are similar to those measured
in blue-shifted circumstellar material from the symbiotic nova RS Oph, as well
as a few Type Ia supernovae that show evidence of circumstellar material, such
as 2006X, 2007le, and PTF 11kx. Supernovae occurring in such systems may not
show circumstellar material interaction until the inner nova shell gets hit by
the supernova ejecta, days to months after the explosion.
