  We report the discovery of a sub-Jupiter mass exoplanet transiting a
magnitude V=11.7 host star 1SWASP J030928.54+304024.7. A simultaneous fit to
the transit photometry and radial-velocity measurements yield a planet mass
M_p=0.53+-0.07M_J, radius R_p=0.91^{+0.06}_{-0.03}R_J and an orbital period of
3.722465^{+0.000006}_{-0.000008} days. The host star is of spectral type K3V,
with a spectral analysis yielding an effective temperature of 4800+-100K and
log g=4.45+-0.2. It is amongst the smallest, least massive and lowest
luminosity stars known to harbour a transiting exoplanet. WASP-11b is the third
least strongly irradiated transiting exoplanet discovered to date, experiencing
an incident flux F_p=1.9x10^8 erg s^{-1} cm^{-2} and having an equilibrium
temperature T_eq=960+-70K.
