Context : 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 .

Aims : 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 .

Methods : 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 ) .
In this first systematic study of its kind , we employed the 104-cm Sampurnanand telescope , the 201-cm Himalayan Chandra telescope and the 200-cm IUCAA-Girawali Observatory telescope , to carry out R -band monitoring on a total of 47 nights .
Using the CCD as an N-star photometer to densely monitor each quasar for a minimum duration of about 4 hours per night , INOV exceeding \sim 1–2 per cent could be reliably detected .
Combining these INOV data with those taken from the literature , after ensuring conformity with the basic selection criteria we adopted for the 16 CDQs monitored by us , we were able to increase the sample size to 21 CDQs ( 12 LPCDQs and 9 HPCDQs ) monitored on a total of 73 nights .

Results : 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 , with 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 .

Conclusions : 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 .
Moreover , the correlation is found to persist for many years after the polarization measurements were made .
Some possible implications of this result are pointed out , particularly in the context of the recently detected rapid \gamma -ray variability of blazars .