  We develop a new perturbation method for determining a class of time transfer
functions in a stationary spacetime when its metric is a small deformation of a
background metric for which the time transfer functions are known in a closed
form. The perturbation terms are expressed as line integrals along the null
geodesic paths of the background metric. Unlike what happens with the other
procedures proposed until now, the time transfer functions obtained in this way
are completely free of unbounded terms and do not generate any enhancement in
the light travel time. Our procedure proves to be very efficient when the
background metric is a linearized Schwarzschild-like metric. Its application to
an isolated body slowly rotating about an axis of symmetry leads to integrals
which can be calculated with any symbolic computer program. Explicit
expressions are obtained for the mass dipole and quadrupole moments and for the
leading gravitomagnetic term induced by the spin of the body. A brief numerical
discussion is given for the 2002 Cassini experiment.
