  We report the discovery of a transiting planet with an orbital period of
1.36d orbiting the brighter component of the visual binary star BD -07 436. The
host star, WASP-77A, is a moderately bright G8V star (V=10.3) with a
metallicity close to solar ([Fe/H]= 0.0 +- 0.1). The companion star, WASP-77B,
is a K-dwarf approximately 2 magnitudes fainter at a separation of
approximately 3arcsec. The spectrum of WASP-77A shows emission in the cores of
the Ca II H and K lines indicative of moderate chromospheric activity. The WASP
lightcurves show photometric variability with a period of 15.3 days and an
amplitude of about 0.3% that is probably due to the magnetic activity of the
host star. We use an analysis of the combined photometric and spectroscopic
data to derive the mass and radius of the planet (1.76+-0.06MJup,
1.21+-0.02RJup). The age of WASP-77A estimated from its rotation rate (~1 Gyr)
agrees with the age estimated in a similar way for WASP-77B (~0.6 Gyr) but is
in poor agreement with the age inferred by comparing its effective temperature
and density to stellar models (~8 Gyr). Follow-up observations of WASP-77 Ab
will make a useful contribution to our understanding of the influence of
binarity and host star activity on the properties of hot Jupiters.
