Kepler's original form of his 3rd law failed to account for the effect of the center of mass. This was understandable: the Sun is so massive compared with the planets that the center of mass of the Sun and a planet lies inside the Sun. Thus, this effect was not detectable using the data available to Kepler.
Here are two Java applets that illustrate this:
1. Kepler's Original Law
2. Modified Kepler's Law
The first illustrates elliptical motion around a fixed Sun. The second illustrates elliptical motion around the center of mass for a binary star (or star-planet) system. In the second one you can vary the masses to see how the center of mass changes. Notice that if one mass is made very large, you recover the original Kepler law of elliptical motion around the (essentially fixed in space) larger mass.