To search for optical variability on a wide range of time scales, we have
carried out photometric monitoring of two flat spectrum radi
o quasars, 3C 454.3 and 3C
279, plus one BL Lac, S5 0716+714, all of which have been exhib
iting remarkably high
activity and pronounced variability at all wavelengths. CC
D magnitudes in B, V, R and
I pass-bands were determined for
∼
7000 new optical observations from 114 nights made
during 2011 – 2014, with an average length of
∼
4 h each, at seven optical telescopes:
four in Bulgaria, one in Greece, and two in India. We measured
multiband optical flux
and colour variations on diverse timescales. Discrete corr
elation functions were com-
puted among B, V, R, and I observations, to search for any time
delays. We found weak
correlations in some cases with no significant time lags. The
structure function method
was used to estimate any characteristic time-scales of vari
ability. We also investigated
the spectral energy distribution of the three blazars using
B, V, R, I, J and K pass-band
data. We found that the sources almost always follows a bluer
-when-brighter trend. We
discuss possible physical causes of the observed spectral v
ariability.

Project 2
: We monitored BL Lacertae for 13 nights in optical B, V, R, and
I bands
during October and November 2014 including quasi-simultan
eous observations in V and
R bands using two optical telescopes in India. We have studie
d multi-band optical flux
variations, colour variation and spectral changes in this b
lazar. Source was found to be
active during the whole monitoring period and showed signifi
cant intraday variability
on 3 nights in V and R filters while displayed hints of variabil
ity on 6 other dates in R
passband and on 2 nights in V filter. From the colour-magnitud
e analysis of the source
we found that the spectra of the target gets flatter as it becom
es brighter on intra-night
timescale. Using discrete correlation technique, we found
that intraday light curves in
both V and R filters are almost consistent and well correlated
with each other. We also
generated spectral energy distribution (SED) of the target
using the B, V, R, and I data
sets for all 13 nights which could help us investigate the phy
sical process responsible for
the observed variations in BL Lacertae objects. We also disc
uss possible physical causes
of the observed spectral variability.

Project 3 (ongoing)
: In this project, we study variability of
∼
15 blazars on all
three timescales using Optical/NIR photometric observati
ons. We found significant flux
variations in most of them. We have measured the multiband op
tical flux and colour
variations in these blazars on intra-day and short-term tim
escales. We also plan to
investigate the spectral slope variability and possible co
rrelations between colour and
magnitude to differentiate between FSRQs and BL Lacertae obj
ects. We also plan to
do cross-correlation analysis to search for time delays.