A cross-calibration between Tycho-2 photometry and HST spectrophotometry
  I show that Tycho-2 photometry and HST spectrophotometry are accurate and stable enough to obtain a precise cross-calibration by analyzing a well-calibrated sample of 256 stars observed with both Hipparcos and HST. Based on this analysis, I obtain the following photometric zero points in the Vega magnitude system for Tycho-2: 0.020+-0.001 (B_T-V_T), 0.078+-0.009 (B_T), and 0.058+-0.009 (V_T). 
J. Ma\'iz-Apell\'aniz (STScI)