A Search for Mass Loss on the Cepheid Instability Strip using HI 21-cm
  Line Observations
  We present the results of a search for HI 21-cm line emission from the
circumstellar environments of four Galactic Cepheids (RS Pup, X Cyg, $\zeta$
Gem, and T Mon) based on observations with the Karl G. Jansky Very Large Array.
The observations were aimed at detecting gas associated with previous or
ongoing mass loss. Near the long-period Cepheid T Mon, we report the detection
of a partial shell-like structure whose properties appear consistent with
originating from an earlier epoch of Cepheid mass loss. At the distance of T
Mon, the nebula would have a mass (HI+He) of $\sim0.5M_{\odot}$, or $\sim$6\%
of the stellar mass. Assuming that one-third of the nebular mass comprises
swept-up interstellar gas, we estimate an implied mass-loss rate of ${\dot
  M}\sim (0.6-2)\times10^{-5} M_{\odot}$ yr$^{-1}$. No clear signatures of
circumstellar emission were found toward $\zeta$ Gem, RS Pup, or X Cyg,
although in each case, line-of-sight confusion compromised portions of the
spectral band. For the undetected stars, we derive model-dependent $3\sigma$
upper limits on the mass-loss rates, averaged over their lifetimes on the
instability strip, of $<(0.3-6)\times10^{-6} M_{\odot}$ yr$^{-1}$ and estimate
the total amount of mass lost to be less than a few per cent of the stellar
mass.
