  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).
