  The frequencies of quasi-periodic oscillations around neutron stars are
believed to be related to characteristic frequencies in the gravitational
fields of the compact objects. In different variability models, these include
the Keplerian, epicyclic, and Lense-Thirring frequencies, which depend mostly
on the properties of the stellar spacetimes. We argue that quasi-periodic
oscillations in the X-ray flux of neutron stars can be used to map the external
spacetimes of the compact objects and, therefore, lead to direct tests of
general relativity in the strong-field regime. In particular, we show that
particular extensions of General Relativity, in which the gravitational force
felt by matter is mediated by both a rank-two tensor and a scalar field, can be
constrained by current observations.
