Differential Astrometry of Subarcsecond Scale Binaries at the Palomar Testbed Interferometer

We have used the Palomar Testbed Interferometer to perform very high precision differential astrometry on the 0!!25 separation binary star HD 171779. In 70 minutes of observation, we achieve a measurement uncertainty of ~9 mu as in one axis, consistent with theoretical expectations. Night-to-night repeatability over four nights is at the level of 16 mu as. This method of very narrow angle astrometry may be extremely useful for searching for planets with masses as small as 0.5M sub(J) around a previously neglected class of stars-so-called speckle binaries. It will also provide measurements of stellar parameters such as masses and distances, useful for constraining stellar models at the 10 super(-3) level.