Does a double star system have more mass than its constituents? According to Einstein, energy is equal to mass. Consider a planet that is in gravitational attraction to two stars. Normally I would say that the gravitational attraction is proportional to the masses of the two stars. But if they are orbiting each other, they possess energy.
Is it correct to say that this star system therefore has a stronger gravitational pull that is greater than just the two added masses of the stars?
 A: A double star has less energy than if the two stars were separated.
It is fairly easy to see why this is. If you have two stars orbiting each other you would need to add energy to separate them. That is, assuming you had some form of Star Trek-esque tractor beam you'd have to use that to grab the stars and physically pull them away from each other. Then you'd be putting work into the system and that added work means the two separated stars have a greater combined energy than the original double star system.
That means the gravitational field of a double star system is slightly smaller than you'd expect from the masses of the two stars, though in practice the difference is far too small every to be measured.
A: Your question is not really different from a proton and an electron joining together to form a hydrogen atom.
When such a combination takes place approximately $13.6\,\rm eV$ of energy is released and the mass of a hydrogen atom is less by the mass equivalent of $13.6\,\rm eV$ $(E=mc^2)$ than the combined mass of an isolate proton and an isolated electron.
So if you found the mass of two stars separately and then arranged for them to be in a bound state orbiting each other, the system of two orbiting stars would have a mass which was less than the combined masses of the two individual stars.
A: The sum of the kinetic and potential energy of a bound system is negative. This must be the case, because you would have to inject more energy into the system to separate the components to infinity.
Therefore the "gravitational mass" of the binary - what you would measure with another orbiting test mass at greater distance - would be less than you would expect from the sum of all the masses of the components of the system measured when they are far apart and stationary.
Whether this is an important effect (i.e. the relative size of the correction) can be judged from the ratio of the absolute value of binding energy to the rest mass energy:
$$\alpha \simeq \frac{GM_1M_2}{2Rc^2(M_1+M_2)}\ ,$$
where $R$ is the separation and the result is exact for a circular orbit.
The effect can be important in binaries featuring compact stars (e.g. neutron stars) that have stellar masses and where $R$ can be quite small - the ratio above is a few per cent for a pair of neutron stars separated by 30 km.
As an aside, this consideration also applies to single stars, where the sum of their internal kinetic energy and gravitational potential energy is also negative. Again, this is important in white dwarf and neutron star physics.
