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 \sim 0.5 M _ { \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 ) \times 10 ^ { -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 ~ { } \hbox to 0.0 pt { $ < $ } { \lower 4.3 pt \hbox { $ \sim$ } } ( 0.3 - 6 ) \times 10 ^ { -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 .