  M4 and NGC 6397 are two very similar galactic globular clusters, which differ
mainly in their surface brightness profile. M4 has a classic King-like profile,
whereas NGC 6397 has a more concentrated profile, which is often interpreted as
that of a post-core collapse cluster. Heggie & Giersz (2008), however, found
that M4 is also a post-core collapse cluster, and Giersz & Heggie (2009)
concluded that the main reason for the difference between the two surface
brightness profiles is fluctuations. This conclusion was reached on the basis
of Monte Carlo models, however, and in the present Letter we verify that
similar fluctuations occur in N-body models. The models were initialised by
generating initial conditions from the Monte Carlo model of NGC 6397 at the
simulated age of 12Gyr, and one was followed for 1Gyr. The new models help to
clarify the nature of the fluctuations, which have the nature of semi-regular
oscillations with a time scale of order 10^8yr. They are influenced by the
dynamical role which is played by primordial binaries in the evolution of the
core.
