ESO AND NOT PHOTOMETRIC MONITORING OF THE CLOVERLEAF QUASAR

The Cloverleaf quasar, H1413+117, has been photometrically monitored a t ESO (La Silla, Chile) and with the NOT (La Palma, Spain) during the period 1987-1994. All good quality CCD frames have been successfully a nalysed using two independent methods (i.e. an automatic image decompo sition technique and an interactive CLEAN algorithm). The photometric results from the two methods are found to be very similar, and they sh ow that the four lensed QSO images vary significantly in brightness (b y up to 0.45 mag), nearly in parallel. The lightcurve of the D compone nt presents some slight departures from the general trend which are ve ry likely caused by micro-lensing effects. Upper limits, at the 99% co nfidence level, of 150 days on the absolute value for the time delays between the photometric lightcurves of this quadruply imaged variable QSO, are derived. This is unfortunately too large to constrain the len s model but there is little doubt that a better sampling of the lightc urves should allow to accurately derive these time delays. Pending a d irect detection of the lensing galaxy (position and redshift), this sy stem thus constitutes another good candidate for a direct and independ ent determination of the Hubble parameter.