Clocks around Sgr A*
  The S stars near the Galactic centre and any pulsars that may be on similar
orbits, can be modelled in a unified way as clocks orbiting a black hole, and
hence are potential probes of relativistic effects, including black hole spin.
The high eccentricities of many S stars mean that relativistic effects peak
strongly around pericentre; for example, orbit precession is not a smooth
effect but almost a kick at pericentre. We argue that concentration around
pericentre will be an advantage when analysing redshift or pulse-arrival data
to measure relativistic effects, because cumulative precession will be drowned
out by Newtonian perturbations from other mass in the Galactic-centre region.
Wavelet decomposition may be a way to disentangle relativistic effects from
Newton perturbations. Assuming a plausible model for Newtonian perturbations on
S2, relativity appears to be strongest in a two-year interval around
pericentre, in wavelet modes of timescale approximately 6 months.
