  We present high-precision photometry of five consecutive transits of WASP-18,
an extrasolar planetary system with one of the shortest orbital periods known.
Through the use of telescope defocussing we achieve a photometric precision of
0.47 to 0.83 mmag per observation over complete transit events. The data are
analysed using the JKTEBOP code and three different sets of stellar
evolutionary models. We find the mass and radius of the planet to be M_b =
10.43 +/- 0.30 +/- 0.24 Mjup R_b = 1.165 +/- 0.055 +/- 0.014 Rjup (statistical
and systematic errors) respectively. The systematic errors in the orbital
separation and the stellar and planetary masses, arising from the use of
theoretical predictions, are of a similar size to the statistical errors and
set a limit on our understanding of the WASP-18 system. We point out that seven
of the nine known massive transiting planets (M_b > 3 Mjup) have eccentric
orbits, whereas significant orbital eccentricity has been detected for only
four of the 46 less massive planets. This may indicate that there are two
different populations of transiting planets, but could also be explained by
observational biases. Further radial velocity observations of low-mass planets
will make it possible to choose between these two scenarios.
