Intra-night optical variability of core dominated radio quasars: the
  role of optical polarization
  {Abridged} Rapid variations in optical flux are seen in many quasars and all
blazars. The amount of variability in different classes of Active Galactic
Nuclei has been studied extensively but many questions remain unanswered. We
present the results of a long-term programme to investigate the intra-night
optical variability (INOV) of powerful flat spectrum radio core-dominated
quasars (CDQs), with a focus on probing the relationship of INOV to the degree
of optical polarization. We observed a sample of 16 bright CDQs showing strong
broad optical emission lines and consisting of both high and low optical
polarization quasars (HPCDQs and LPCDQs). We employed ARIES, IIA, IGO
telescopes, to carry out {\it R}-band monitoring on a total of 47 nights.
Combining these INOV data with those taken from the literature, we were able to
increase the sample size to 21 CDQs(12 LPCDQs and 9 HPCDQs) monitored on a
total of 73 nights. As the existence of a prominent flat-spectrum radio core
signifies that strong relativistic beaming is present in all these CDQs, the
definitions of the two sets differ primarily in fractional optical
polarization, the LPCDQs showing a very low median$ P_{op} \simeq$ 0.4 per
cent.
  Our study yields an INOV duty cycle (DC) of $\sim$28 per cent for the LPCDQs
and $\sim 68$ percent for HPCDQs.
  If only strong INOV with fractional amplitude above 3 per cent is considered,
the corresponding DCs are $\sim$ 7 per cent and $\sim$ 40 per cent,
respectively.From this strong contrast between the two classes of luminous,
relativistically beamed quasars, it is apparent that relativistic beaming is
normally not a sufficient condition for strong INOV and a high optical
polarization is the other necessary condition.
