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Blazars are known to show flux variations over a range of energies from lowenergy radio to high energy gamma-rays. Cross-correlation analysis of theoptical and $\gamma$-ray light curves in blazars shows that flux variations aregenerally correlated in both bands, however, there are exceptions. We exploredthis optical-GeV connection in four flat spectrum radio quasars (FSRQs) by asystematic investigation of their long term optical and $\gamma$-ray lightcurves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510$-$089and CTA 102 we noticed different behaviours between the optical and GeV fluxvariations. We found instances when (i) the optical and GeV flux variations areclosely correlated (ii) there are optical flares without $\gamma$-raycounterparts and (iii) $\gamma$-ray flares without optical counterparts. Tounderstand these diverse behaviours, we carried out broad band spectral energydistribution (SED) modelling of the sources at different epochs using aone-zone leptonic emission model. The optical-UV emission is found to bedominated by emission from the accretion disk in the sources PKS 1510$-$089,CTA 102 and 3C 273, while in 3C 279, the synchrotron radiation from the jetdominates the optical-UV emission. Our SED analysis indicates that (i)correlated optical and $\gamma$-ray flux variations are caused by changes inthe bulk Lorentz factor ($\Gamma$), (ii) $\gamma$-ray flares without opticalcounterparts are due to increase in $\Gamma$ and/or the electron energy densityand (iii) an optical flare without $\gamma$-ray counterpart is due to increasein the magnetic field strength.

Correlation between optical and $\gamma$-ray flux variations in bright flat spectrum radio quasars