  We report the discovery of WASP-166b, a super-Neptune planet with a mass of
0.1 Mjup (1.9 Mnep) and a bloated radius of 0.63 Rjup. It transits a V = 9.36,
F9V star in a 5.44-d orbit that is aligned with the stellar rotation axis
(sky-projected obliquity angle lambda = 3 +/- 5 degrees). Variations in the
radial-velocity measurements are likely the result of magnetic activity over a
12-d stellar rotation period. WASP-166b appears to be a rare object within the
``Neptune desert''.
