A stellar companion in the HD 189733 system with a known transiting extrasolar planet

We show that the very close-by (19 pc) K0 star HD 189733, already found to be orbited by a transiting giant planet, is the primary of a double star system, with the secondary being a mid-M dwarf with projected separation of about 216 AU from the primary. This conclusion is based on astrometry, proper-motion and radial velocity measurements, spectral type determination, and photometry. We also detect differential proper motion of the secondary. The data appear consistent with the secondary's orbiting the primary in a clockwise orbit, lying nearly in the plane of the sky (i.e., nearly perpendicular to the orbital plane of the transiting planet), and with period of about 3200 years.