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.
@article{arxiv.0809.4597,
title = {The sub-Jupiter mass transiting exoplanet WASP-11b},
author = {R. G. West and A. Collier Cameron and L. Hebb and Y. C. Joshi and D. Pollacco and E. Simpson and I. Skillen and H. C. Stempels and P. J. Wheatley and D. Wilson and D. Anderson and S. Bentley and F. Bouchy and B. Enoch and N. Gibson and G. Hébrard and C. Hellier and B. Loeillet and M. Mayor and P. Maxted and I. McDonald and C. Moutou and F. Pont and D. Queloz and A. M. S. Smith and B. Smalley and R. A. Street and S. Udry},
journal= {arXiv preprint arXiv:0809.4597},
year = {2015}
}