We present MERLIN maps of the 22-GHz H _ { 2 } O masers around four low-mass late-type stars ( IK Tau U Ori , RT Vir and U Her ) , made with an angular resolution of \sim 15 milliarcsec and a velocity resolution of 0.1 km s ^ { -1 } .
The H _ { 2 } O masers are found in thick expanding shells with inner radii \sim 6 to 16 au and outer radii four times larger .
The expansion velocity increases radially through the H _ { 2 } O maser regions , with logarithmic velocity gradients of 0.5–0.9 .
IK Tau and RT Vir have well-filled H _ { 2 } O maser shells with a spatial offset between the near and far sides of the shell , which suggests that the masers are distributed in oblate spheroids inclined to the line of sight .
U Ori and U Her have elongated poorly-filled shells with indications that the masers at the inner edge have been compressed by shocks ; these stars also show OH maser flares .
MERLIN resolves individual maser clouds , which have diameters of 2 – 4 au and filling factors of only \sim 0.01 with respect to the whole H _ { 2 } O maser shells .
The CSE velocity structure gives additional evidence the maser clouds are density bounded .
Masing clouds can be identified over a similar timescale to their sound crossing time ( \sim 2 yr ) but not longer .
The sizes and observed lifetimes of these clouds are an order of magnitude smaller than those around red supergiants , similar to the ratio of low-mass : high-mass stellar masses and sizes .
This suggests that cloud size is determined by stellar properties , not local physical phenomena in the wind .