  We report on Kepler photometry of the hot sdB star B4 in the open cluster NGC
6791. We confirm that B4 is a reflection effect binary with an sdB component
and a low-mass main sequence companion with a circular 0.3985 d orbit. The sdB
star is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384 s
to 7643 s. Several of the pulsation modes show symmetric splitting by 0.62
microHz. Attributing this to rotational splitting, we conclude that the sdB
component has a rotation period of approximately 9.63 d, indicating that tidal
synchronization has not been achieved in this system. Comparison with
theoretical synchronization time provides a discriminant between various
theoretical models.
