  We report the discovery of a massive (Mp = 9.04+/-0.50 MJup) planet
transiting the bright (V = 8.7) F8 star HD 147506, with an orbital period of
5.63341+/-0.00013 days and an eccentricity of e = 0.520+/-0.010. From the
transit light curve we determine that the radius of the planet is Rp =
0.982^{+0.038}_{0.105}RJup. HD 147506b (also coined HAT-P-2b) has a mass about
9 times the average mass of previously-known transiting exoplanets, and a
density of rho = 11.9 g cm-3, greater than that of rocky planets like the
Earth. Its mass and radius are marginally consistent with theories of structure
of massive giant planets composed of pure H and He, and may require a large
(~100 Earth mass) core to account for. The high eccentricity causes a 9-fold
variation of insolation of the planet between peri- and apastron. Using
follow-up photometry, we find that the center of transit is Tmid =
2,454,212.8559 +/- 0.0007 (HJD), and the transit duration is 0.177 +/-0.002 d.
