The Cloverleaf quasar, H1413+117, was discovered to be a quadruply imaged QSO by Magain et al. (1988). Since then, it has been photometrically monitored essentially as part of the ESO Key-Program Gravitational Lensing (La Silla) and as part of the Gravitational Lens Monitoring Program at the Nordic Optical Telescope (NOT) on the island of La Palma. This quasar, at a redshift of 2.55, and with an apparent visual magnitude of 17, is one of the brightest members of the class of broad absorption line (BAL) QSOs. In addition, its spectrum shows at least three narrow absorption line systems at redshifts z=1.44, 1.66 and 2.07. These are attributed to intervening gas clouds (Hazard et al. 1984; Drew & Boksenberg 1984; Turnshek et al. 1988), possibly associated with the lens(es) (Magain et al. 1988). Imaging of H1413+117 shows that the four lensed components are separated by approximately one arcsecond (''). However, no lensing object has yet been detected. The spectra of two of the four images have been found by Magain et al. (1988) to be identical, except for narrow absorption line systems at z=1.44 and 1.66, which are much stronger in component B than in C. Two-dimensional spectroscopy by Angonin et al. (1990) also shows that the spectrum of component D has smaller values for the emission lines/continuum ratios and a larger equivalent width for the absorption features in the P Cygni profiles, compared to the other three lensed QSO components. This is most likely caused by micro-lensing effects. Observations from the VLA at NRAO show radio counterparts for the four QSO components, as well as an additional strong radio source between images B and D (Kayser et al. 1990), but no radio source has been found that can be associated with a lensing object.
As part of the Gravitational Lensing ESO Key-Program, CCD photometry of H1413+117 has been carried out with the aim of detecting photometric variability of the QSO itself, and thereby determining the time delay(s), or the signature of micro-lensing effects. Preliminary results for the four images have been presented by Arnould et al. (1993) and by Remy et al. (1996). These authors report that from 1987 to 1993 the four lensed components apparently display brightness variations quasi-simultaneously and in parallel. They set an upper limit for the time delays of roughly a few months. In addition to these variations, which are ascribed to the source, the D component has been found to show extra light variations with respect to the other components. These extra variations have been interpreted as being possibly induced by micro-lensing effects.
The astrophysical and cosmological justifications for conducting photometric monitoring programs of gravitational lens systems are summarized in Refsdal & Surdej (1992, 1994). The importance of recording well sampled and accurate lightcurves for such systems can hardly be overestimated. The ESO Key-Program observations took place during the period 1987-1993 (see Table 1, accessible on the WWW).
The original plans were to monitor the known gravitational lens systems
Q2237+0305 (the Einstein Cross), UM425,
(ESO GL1, Surdej et al. 1987,
1988) and H1413+117 (the Cloverleaf), at weekly intervals.
However, because of unfavorable weather conditions, unexpected technical
problems or sometimes the non-availability of a direct CCD camera at the
foci of the ESO telescopes, the sampling of the photometric lightcurves has
not been as frequent as initially expected. After some preliminary
observations in 1990, a program was also started at the NOT, for monitoring
the four gravitational lens systems 
, Q0957+561 (the
"classical" Double Quasar), H1413+117 and Q2237+0305. Although
observations were similarly aimed at weekly intervals, several large gaps in
the NOT data have occurred for the same reasons as those mentioned above.
Results for the Einstein Cross have been published by Østensen et
al. (1996), for UM425 by Courbin et al.
(1995) and for by Hjorth et al.
(1996) and Daulie et al. (1993).
In this paper we present results for the Cloverleaf, obtained using two independent methods of analysis, combining all ESO and NOT observations.