### 4.1 Post-Keplerian timing parameters

In any given theory of gravity, the post-Keplerian (PK) parameters can be written as functions of the
pulsar and companion star masses and the Keplerian parameters. As the two stellar masses are the only
unknowns in the description of the orbit, it follows that measurement of any two PK parameters will yield
the two masses, and that measurement of three or more PK parameters will over-determine the problem
and allow for self-consistency checks. It is this test for internal consistency among the PK parameters that
forms the basis of the classic tests of strong-field gravity. It should be noted that the basic
Keplerian orbital parameters are well-measured and can effectively be treated as constants
here.
In general relativity, the equations describing the PK parameters in terms of the stellar masses are
(see [35, 132, 45]):

where , and . Other theories of gravity, such
as those with one or more scalar parameters in addition to a tensor component, will have somewhat
different mass dependencies for these parameters. Some specific examples will be discussed in Section 4.4
below.