I'm not sure if this was mentioned in any other threads here; I could not find any reference to it.

A few weeks ago, a double pulsar was discovered by Andrew Lyne et al.; a binary pulsar where both members are radio-visible pulsars:

J0737-3039A - 23 ms
J0737-3039B - 2.77 s

with an orbit period of around 2.4 hours. The orbit's semimajor axis is around 800,000 km, and its eccentricity around 0.0878.

B was observed to eclipse A, and A apparently modulates B's emissions, changing B's observed luminosity and pulse shape.

I propose names for A and B, these Sesame-Street muppets:
A: Ernie (has a short, broad head)
B: Bert (has a tall, narrow head)
However, those names are likely to run into copyright problems...

Binary pulsars in general are good tests of relativistic-gravity theories, and this double pulsar should offer some especially good tests, since both pulsars being visible increases the supply of observable parameters. Thus making them something like eclipsing double-line binary stars, while previous binary pulsars have been something like single-line binary stars.

In particular, the Newtonian-limit time delays across the orbits fixes the mass ratio very precisely, and the eclipses suggest that the orbit's inclination is close to 90 deg (edge-on).

Relativistic effects have also been observed for A, though not yet for B, like:
Redshift due to motion and the other object's gravitational field
Shapiro extra time delay near a massive object
Precession of the orbit's periapsis

Assuming the correctness of general relativity and that other effects are negligible, some very precise mass estimates can be obtained:
A: 1.34 solar masses
B: 1.25 solar masses

These are a little less than the Chandrasekhar limit; could the difference be partially due to gravitational binding energy? Or gravitational-collapse details?

However, alternative theories like the Generalized Brans-Dicke theory predict a metric that looks much like GR's "post-Newtonian" metric, but with fudge factors, usually called gamma and beta. GR predicts that both are 1; GBD in general predicts non-unity values, but values that can be made arbitrarily close to 1 with appropriate parameter selection, making the theory almost indistinguishable from GR. And GBD-like theories may possibly be a low-energy limit of superstrings instead of pure GR.

The aforementioned redshift can be derived from the Equivalence Principle, so they should be the same in any metric theory of gravity (GR, GBD, etc.).

The Shapiro time delay (and deflection of light) is (1+gamma)/2 * the GR value.
It has two parameters, "r" (the delay value is proportional to it) and "s" (a shape value, equal to sin(inclination))

The precession is (2+2*gamma-beta)/3 * the GR value

The appropriate terms of the post-Newtonian metric are:

g_00 = - (1 + 2*V + 2*beta*V^2)
g_0i = 0
g_ij = delta_ij * (1 - 2*gamma*V)

where c = 1 units are used, velocity-dependent effects and some others are ignored for simplicity, and V is the Newtonian gravitational potential, - sum(GM/r). This metric is, however, correct for a single stationary source.

From Solar-System observations and experiments, however, both gamma and beta agree with GR to within a few times 0.1%.

Finally, it will be interesting to find the orbit-period rate of change, which is most likely due to gravitational radiation. Most alternative theories of gravity predict that the gravitational-radiation-source mass will be different from the inertial mass, on account of different contributions from their gravitational-potential energy, while GR predicts that they will be equal. Alternative theories thus usually predict gravitational-dipole radiation in addition to GR's gravitational-quadrupole radiation.

A gravitational dipole would be zero for two identical objects. But in this case, the two pulsars differ in mass by 10%, perhaps enough to make a gravitational-dipole effect noticeable -- if it exists. In any case, the double pulsar's orbit parameters are determined well enough to make precise predictions possible of how much gravitational radiation one can expect -- without assuming the GR values of the post-Newtonian parameters beta and gamma.


Refs:
PhysicsWeb article
CSIRO article
Science magazine report
Arxiv preprint of that report (PDF)
Nature magazine report
Clifford Will's article on Tests of General Relativity
Testing General Relativity with Pulsar Timing, by Ingrid Stairs
Binary and Millisecond Pulsars at the New Millennium, by Duncan Lorimer