Context :

Aims : The rapidly rotating Be star $ φ $ Persei was spun up by mass and angular momentum transfer from a now stripped-down , hot subdwarf companion .
Here we present the first high angular resolution images of $ φ $ Persei made possible by new capabilities in long-baseline interferometry at near-IR and visible wavelengths .
We analyzed these images to search for the companion , to determine the binary orbit , stellar masses , and fluxes , and to examine the geometrical and kinematical properties of the outflowing disk surrounding the Be star .

Methods : We observed $ φ $ Persei with the MIRC and VEGA instruments of the CHARA Array .
MIRC was operated in six-telescope mode , whereas VEGA was configured in four-telescope mode with a change of quadruplets of telescopes during two nights to improve the ( u,v ) plane coverage .
Additional MIRC-only observations were performed to track the orbital motion of the companion , and these were fit together with new and existing radial velocity measurements of both stars to derive the complete orbital elements and distance .
We also used the MIRC data to reconstruct an image of the Be disk in the near-IR H -band .
VEGA visible broadband and spectro-interferometric H \alpha observations were fit with analytical models for the Be star and disk , and image reconstruction was performed on the spectrally resolved H \alpha emission line data .

Results : The hot subdwarf companion is clearly detected in the near-IR data at each epoch of observation with a flux contribution of 1.5 % in the H band , and restricted fits indicate that its flux contribution rises to 3.3 % in the visible .
A new binary orbital solution is determined by combining the astrometric and radial velocity measurements .
The derived stellar masses are 9.6 \pm 0.3 M _ { \odot } and 1.2 \pm 0.2 M _ { \odot } for the Be primary and subdwarf secondary , respectively .
The inferred distance ( 186 \pm 3 pc ) , kinematical properties , and evolutionary state are consistent with membership of $ φ $ Persei in the $ α $ ~Per cluster .
From the cluster age we deduce significant constraints on the initial masses and evolutionary mass transfer processes that transformed the $ φ $ Persei binary system .
The interferometric data place strong constraints on the Be disk elongation , orientation , and kinematics , and the disk angular momentum vector is coaligned with and has the same sense of rotation as the orbital angular momentum vector .
The VEGA visible continuum data indicate an elongated shape for the Be star itself , due to the combined effects of rapid rotation , partial obscuration of the photosphere by the circumstellar disk , and flux from the bright inner disk .

Conclusions :