What physical law, in its general form, connects the sum of the masses to the orbital period and semi-major axis in a binary system?

Answer

Newtonian form of Kepler's Third Law

The relationship used is a crucial extension of Kepler's Third Law, refined by Newton's Law of Universal Gravitation, to incorporate both masses in the system.

What physical law, in its general form, connects the sum of the masses to the orbital period and semi-major axis in a binary system?
AstronomerLawstarmassbinary system