  High angular resolution observations at optical wavelengths provide valuable
insights in stellar astrophysics, directly measuring fundamental stellar
parameters, and probing stellar atmospheres, circumstellar disks, elongation of
rapidly rotating stars, and pulsations of Cepheid variable stars. The angular
size of most stars are of order one milli-arcsecond or less, and to spatially
resolve stellar disks and features at this scale requires an optical
interferometer using an array of telescopes with baselines on the order of
hundreds of meters. We report on the successful implementation of a stellar
intensity interferometry system developed for the four VERITAS imaging
atmospheric-Cherenkov telescopes. The system was used to measure the angular
diameter of the two sub-mas stars $\beta$ Canis Majoris and $\epsilon$ Orionis
with a precision better than 5%. The system utilizes an off-line approach where
starlight intensity fluctuations recorded at each telescope are correlated
post-observation. The technique can be readily scaled onto tens to hundreds of
telescopes, providing a capability that has proven technically challenging to
current generation optical amplitude interferometry observatories. This work
demonstrates the feasibility of performing astrophysical measurements with
imaging atmospheric-Cherenkov telescope arrays as intensity interferometers and
the promise for integrating an intensity interferometry system within future
observatories such as the Cherenkov Telescope Array.
