  Observations of the rotation rates of horizontal branch (HB) stars show
puzzling systematics. In particular, cooler HB stars often show rapid rotation
(with velocities in excess of 10 km/s), while hotter HB stars typically show
much smaller rotation velocities. Simple models of angular momentum evolution
of stars from the main sequence through the red giant branch fail to explain
these effects. In general, evolutionary models in all cases preserve a rapidly
rotating core. The observed angular velocities of HB stars require that some of
the angular momentum stored in the core reaches the surface.
  To test the idea that HB stars contain such a core, one can appeal to
detailed computations of trace element abundences and rotational mixing.
However, a more direct probe is available to test these limiting cases of
angular momentum evolution. Some of the hottest horizontal branch stars are
members of the pulsating sdB class. They frequently show rich pulsation spectra
characteristic of nonradially pulsating stars. Thus their pulsations probe the
internal rotation of these stars, and should show the effects of rapid rotation
in their cores. Using models of sdB stars that include angular momentum
evolution, we explore this possibility and show that some of the sdB pulsators
may indeed have rapidly rotating cores.
