Some problems in calibrating UBV photometry for effective temperature and surface gravity

The spectrophotometry of 54 A/B stars and of the RR Lyrae variable SU Dra was reduced to the recent recalibration of Vega. By integrating the reduced spectrophotometric results the colors of these stars were calculated. A plot of the computed versus directly observed color indices forms a 45 deg line if b-y is considered. This suggests that the spectrometry, the uvby sensitivity functions and the absolute calibration of vega are correct. However the U-B, B-V data cannot be fitted to a 45 deg line; there are systematic discrepancies. This suggests that at least two of the U, B, V sensitivity functions need further revision and the absolute calibration of UBV colors may contain a systematic error exceeding 10%. The UBV light curve of SU Dra in terms of hydrostatic and radiative equilibrium and LTE model atmospheres were interpreted.