An Interferometric Search for Bright Companions to 51 Pegasi
  We report on a near-infrared, long-baseline interferometric search for luminous companions to the star 51 Pegasi conducted with the Palomar Testbed Interferometer. Our data is completely consistent with a single-star hypothesis. We find no evidence to suggest a luminous companion to 51 Pegasi, and can exclude a companion brighter than a $\Delta$K of 4.27 at the 99% confidence level for the 4.2-day orbital period indicated by spectroscopic measurements. This $\Delta$K corresponds to an upper limit in the companion M$_K$ of 7.30, in turn implying a main-sequence companion mass less than 0.22 M$_{\sun}$. 
A.F. Boden, G.T. van Belle, et al, The PTI Collaboration