Direct detection of exoplanet host star companion γ Cep B and revised masses for both stars and the sub-stellar object

Context. The star gamma Cep is known as a single-lined spectroscopic triple system at a distance of 13.8 pc, composed of a K1 III-IV primary star with V = 3.2 mag, a stellar-mass companion in a 66-67 year orbit (Torres 2007, ApJ, 654, 1095), and a substellar companion with M-p sin i = 1.7 M-Jup that is most likely a planet (Hatzes et al. 2003, ApJ, 599, 1383). Aims. We aim to obtain a first direct detection of the stellar companion, to determine its current orbital position ( for comparison with the spectroscopic and astrometric data), its infrared magnitude and, hence, mass. Methods. We use the Adaptive Optics camera CIAO at the Japanese 8 m telescope Subaru on Mauna Kea, Hawaii, with the semitransparent coronograph to block most of the light from the bright primary. Cep A, and to detect at the same time the faint companion B. In addition, we also used the IR camera Omega Cass at the Calar Alto 3.5 m telescope, Spain, to image. Cep A and B by adding up many very short integrations (without AO). Results. gamma Cep B is clearly detected on our CIAO and Omega Cass images. We use a photometric standard star to determine the magnitude of B after PSF subtraction in the Subaru image, and the magnitude difference between A and B in the Calar Alto images, and find an average value of K = 7.3 +/- 0.2 mag. The separations and position angles between A and B are measured on 15 July 2006 and 11 and 12 Sept. 2006, B is slightly south of west of A. Conclusions. By combining the radial velocity, astrometric, and imaging data, we have refined the binary orbit and determined the dynamical masses of the two stars in the gamma Cep system, namely 1.40 +/- 0.12 M-circle dot for the primary and 0.409 +/- 0.018 M-circle dot for the secondary ( consistent with being a M4 dwarf). We also determine the minimum mass of the sub-stellar companion to be M-p sin i = 1.60 +/- 0.13 M-Jup.