Astrometric binaries are those where one sees only one star, yet the presence of the second star is revealed by orbital motion. The most famous example of an astrometric binary system is the nearby star Sirius. Although that designation has changed since improved telescopes have long since given us images of the companion of Sirius. This page gives some more details and explanations.It is generally true that in a binary star system, it is possible to determine the combined mass of the two stars. This is true of astrometric binaries as well.