  We present the results of a photometric survey of rotation rates in the Coma
Berenices (Melotte 111) open cluster, using data obtained as part of the
SuperWASP exoplanetary transit-search programme. The goal of the Coma survey
was to measure precise rotation periods for main-sequence F, G and K dwarfs in
this intermediate-age (~600 Myr) cluster, and to determine the extent to which
magnetic braking has caused the stellar spin periods to converge. We find a
tight, almost linear relationship between rotation period and J-K colour with a
root-mean square scatter of only 2 percent. The relation is similar to that
seen among F, G and K stars in the Hyades. Such strong convergence can only be
explained if angular momentum is not at present being transferred from a
reservoir in the deep stellar interiors to the surface layers. We conclude that
the coupling timescale for angular momentum transport from a rapidly-spinning
radiative core to the outer convective zone must be substantially shorter than
the cluster age, and that from the age of Coma onward, stars rotate effectively
as solid bodies. The existence of a tight relationship between stellar mass and
rotation period at a given age supports the use of stellar rotation period as
an age indicator in F, G and K stars of Hyades age and older. We demonstrate
that individual stellar ages can be determined within the Coma population with
an internal precision of order 9 percent (RMS), using a standard magnetic
braking law in which rotation period increases with the square root of stellar
age. We find that a slight modification to the magnetic-braking power law, P
proportional to t^0.56, yields rotational and asteroseismological ages in good
agreement for the Sun and other stars of solar age for which p-mode studies and
photometric rotation periods have been published.
