For spectroscopic binaries the orbital velocity curve shows the radial component of the star's velocity as a function of time. The analogous plot for pulsars is the apparent pulse period against time. Two examples are given in Fig. 22 .

Constraints on the mass of the orbiting companion can be placed by combining the projected semi-major axis and the orbital period to obtain the mass function:

where
*G*
is the universal gravitational constant. Assuming a pulsar mass
of
(see below), the mass of the orbiting companion
can be estimated as a function of the (initially unknown) angle
*i*
between the orbital plane and the plane of the sky. The minimum
companion mass
occurs when the orbit is assumed edge-on (). For a random distribution of orbital inclination angles, the
probability of observing a binary system at an angle
*less*
than some value
is
. This implies that the chances of observing a binary system
inclined at an angle
is only 10%; evaluating the companion mass for this inclination
angle
constrains the mass range between
and
at the 90% confidence level.

Binary and Millisecond Pulsars at the New Millennium
Duncan R. Lorimer
http://www.livingreviews.org/lrr-2001-5
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