There is a general consensus that quasars belong to two different
radio populations, radio-quiet quasars (RQQSOs) and radio-loud
quasars. R is usually defined as the ratio of the radio
(6 cm) to the optical (440 nm) flux densities and the
radio-quiet quasars have a value of R<10, while the radio-loud
quasars have R>10 (Kellermann et al. 1989). It is found that
of quasars are in the radio-loud category. An
additional distinction between active galactic nuclei (AGN) with strong
and weak radio sources comes from the observation that radio loud
objects essentially all occur in elliptical galaxies and RQQSOs appear
to reside in galaxies that are dominated by exponential disks. However
the RQQSOs that occur in elliptical host galaxies are in general more
luminous than those that reside in disks
(Taylor et al. 1996).
Little is known about the short-term variability of radio-quiet quasars, because few studies have been carried out (Gopal-Krishna et al. 1993 and 1995; Jang & Miller 1995; Sagar et al. 1996). In contrast blazars display rapid variability in the wavelength range from radio to gamma rays. The blazar class encompasses both optically-violently-variable (OVV) quasars and BL Lac objects and about one quarter of all radio-loud quasars are also in the blazar category (Webb et al. 1988; Pica et al. 1988).
There are many theoretical models which endeavour to explain the large and rapid variability exhibited by blazars and these are usually divided into extrinsic and intrinsic categories. One extrinsic mechanism is microlensing of emission knots in a relativistic jet when they pass behind planets in an intervening galaxy (McBreen & Metcalfe 1987; Gopal-Krishna & Subramanian 1991). The rapid variability from superluminal-microlensing may be responsible for the variability observed in AO 0235+164 (Rabbette et al. 1996) and PKS 0537-441 (Romero et al. 1995). One family of intrinsic models is based on a rotating supermassive black hole which accretes matter from a surrounding accretion disc and ejects two oppositely directed jets. The shocked-jet model involves shocks which move with relativistic speeds along the jet (Qian et al. 1991; Marscher 1980). It is believed the shock propagates along the line of sight, through inhomogeneous, small-scale structures distributed along the jet. These inhomogeneous structures are illuminated, or excited, by the moving relativistic shock, through the amplification of the magnetic field and the acceleration of electrons which causes the variability in polarization and in flux density that are observed over a wide range of frequencies (Hughes et al. 1986). Another family of intrinsic models invokes numerous flares or hotspots in the accretion disk and the corona that is believed to surround the central engine (Wiita et al. 1992; Mangalam & Wiita 1993) and indeed a similar model has been proposed to explain X-ray variations in blazars (Abramowicz et al. 1991). The fact that RQQSOs generally lie on the far-infrared versus radio correlation (Sopp & Alexander 1991) suggest that star formation plays an important role in their radio emission. It has been suggested by Terlevich et al. (1992) that the low values of R in RQQSOs can be explained without jets or accretion discs, by postulating a circumnuclear starburst within a dense, high-metallicity nuclear environment. In this model the optical/UV and bolometric luminosity arises from young stars; the variability comes from cooling instabilities in the shell of compact supernova remnants and supernova flashes. Variability on intranight timescales is however difficult to explain with this model because of the short timescales involved. Furthermore radio-quiet and radio-loud quasars have very different radio power outputs but have similar spectral shapes in the radio region and suggest that a significant fraction of the RQQSOs may be capable of producing powerful radio emission (Barvainis et al. 1996). Kellermann et al. (1994) found possible radio extensions up to about 300 kpc in a few RQQSOs and assert that for at least these few cases, the emission is too large to be starburst related (Stein 1996).
Recently, some evidence suggesting rapid optical variability in the RQQSOs PG 0946+301 and PG 1444+407 was reported by Sagar et al. (1996). They also reported long-term variability for four RQQSOs. Jang & Miller (1995) reported intranight variability for one RQQSO out of a sample of nine sources. Brinkmann et al. (1996) obtained ASCA observations of the radio-quiet, infrared quasar IRAS 13349+2438 and detected substantial X-ray variability on a timescale of only a few hours.
The results of the photometric observations of a sample of mainly high
luminosity and high redshift RQQSOs are presented. The observations
and data reduction are given in Sect. 2. The results including tables
listing the differential photometry and some light curves are
presented in Sect. 3. The discussion and conclusions are given in
Sects. 4 and 5. Section 4 also includes a discussion on two remarkable
RQQSOs, PG 1416-129 and IRAS 13349+2438. CCD
images of the fields containing the radio-quiet quasars and reference
stars used in the differential photometry are also included. A value
of 
and
q0=0.5 has been adopted.