This talk shows an spectrophotometric analysis of HD 209458 and 18 A type stars. We have used absolute flux spectrophotometry obtained with the Hubble Space Telescope  and Kurucz model atmospheres to ascertain the angular diameters (precision of about 1%) and effective temperatures (similar or better than 1%). The same analysis also permits the determination of other stellar parameters such as metallicity and surface gravity. The A type stars were selected from the STIS Next Generation Spectral Library because they match our criteria that their absolute fluxes in the optical are reliable to within 3 per cent by comparing their HST spectra with Kurucz theoretical models. HD 209458 harbors the first transiting planet discovered and it is one of only two systems with absolute stellar and planetary masses derived from high-resolution spectroscopy. We updated the stellar radius by combining the angular diameter with the Gaia parallax and then the planetary radius with unprecedented precision. We have also determined the stellar parameters from stellar evolution models. Finally, our results are compared with those from the literature.