  It has been shown that the luminosity of AGNs and the size of their broad
line region obey a simple relation of the type R=a L^g, from faint Seyfert
nuclei to bright quasars, allowing single-epoch determination of the central
black hole mass M=b L^g D^2 from their luminosity L and width of H_beta
emission line. Adopting this mass determination for cosmological studies
requires the extrapolation to high z and L of a relation whose calibration
relies so far on reverberation mapping measurements performed for L<10^46 erg/s
and z<0.4. We initiated a campaign for the monitoring of a few luminous,
intermediate z quasars whose apparent magnitude V<15.7 allows observations with
a 1.8m telescope, aimed at proving that emission lines vary and respond to
continuum variations even for luminosities >10^47 erg/s, and determining
eventually their M_BH from reverberation mapping. We have repeatedly performed
simultaneous observations of quasars and reference stars to determine relative
variability of continuum and emission lines. We describe the observations and
methods of analysis. For the quasars PG1634+706 and PG1247+268 we obtain
light-curves respectively for CIII], MgII and for CIV, CIII] emission lines
with the relevant continua. During 3.2 years of observation, in the former case
no continuum variability has been detected and the evidence for line
variability is marginal, while in the latter case both continuum and line
variability are detected with high significance and the line variations appear
correlated with continuum variations. The detection of the emission line
variability in a quasar with L~10^47 erg/s encourages the prosecution of the
campaign which should provide a black hole mass estimate in other 5-6 years,
constraining the M_BH-L relation in a poorly explored range of luminosity.
