QSO mass constraints from gravitational lensing studies of quasar pairs. The cases of Q1548+114 A & B and Q1148+0055 A & B

New constraints on the mass of quasars are derived from gravitational lensing studies of the QSO pairs Q1548+114 A & B and Q1148+0055 A & B, for which new ground-based and Hubble Space Telescope (HST) direct imagery have been obtained. In the case of Q1548+114 A & B, QSO A has been resolved into its host galaxy and a close companion. The non-detection with HST of a secondary lensed image of the background QSO in the close vicinity of the foreground one and the modeling of the host of QSO A, of the companion and of field galaxies with Singular Isothermal Spheres (SIS) yield a robust upper limit on the central compact mass of 4.5 10[SUP]11[/SUP] M[SUB]sun[/SUB]. On the other hand, the combined mass of Q1148+0055 B plus host must be smaller than 6.5 10[SUP]11[/SUP] M[SUB]sun[/SUB] since no secondary lensed image has been detected with HST. Photometry and relative astrometry of all the detected objects are reported. Based on data collected with the Hubble Space Telescope and at the European Southern Observatory, La Silla, Chile