We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacs and 1 Flat Spectrum Radio Quasar (FSRQ). We have measured the multiband optical flux and colour variations in these blazars on intra-day and short-term timescales of months and have limited data for 2 more blazars. These photometric observations were made during 2009 to 2011, using six optical telescopes, four in Bulgaria, one in Greece and one in India. On short-term timescales we found significant flux variations in 9 of the sources and colour variations in 3 of them. Intra-day variability was detected on 6 nights for 2 sources out of the 18 nights and 4 sources for which we collected such data. These new optical observations of these blazars plus data from our previous published papers (for 3 more blazars) were used to analyze their spectral flux distributions in the optical frequency range. Our full sample for this purpose includes 6 high-synchrotron-frequency-peaked BL Lacs (HSPs), 3 intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and 6 low-synchrotron-frequency-peaked BL Lacs (LSPs; including both BL Lacs and FSRQs). We also investigated the spectral slope variability and found that the average spectral slopes of LSPs show a good accordance with the Synchrotron Self-Compton (SSC) loss dominated model. Our analysis supports previous studies that found that the spectra of the HSPs and FSRQs have significant additional emission components. The spectra of all these HSPs and LSPs get flatter when they become brighter, while for FSRQs the opposite appears to hold. This supports the hypothesis that there is a significant thermal contribution to the optical spectrum for FSRQs.
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