Ten out of eleven blazars in this paper and in Guibin et al. (1995) are radio loud sources with a radio flat spectrum, other one PKS 1618+177 is a object with a radio steep spectrum. The objects with a flat spectra exhibit day-to-day variability except for PKS 2128-123, and seven such objects show a more rapid variability with timescales of tens minutes to 2 hours. In all probability, variability at the shortest time scales is intrinsic (Wagner et al. 1995 and references therein). Relativistic beaming model can provide an explanation of the variability of blazars. The efficiency for conversion of accreted matter into energy in a spherical, homogeneous, non-relativisticaly beamed source is given by

where , in , is the variation in luminosity
within the time interval in seconds. Relativistic beaming is infered
if the value of exceeds 0.1 (Guilbert et al. 1983). Table 3 (click here) gives the
luminosity and the efficiency for objects with flat spectra except
for PKS 0422+004, assuming *H*_{0} = 50 km and
*q*_{0}=0 and performing the K-correction and correction of the galactic
extinction. The bolometric luminosity *L*, the efficiency and the Doppler factor
are given in Cols. 6, 7 and 8 respectively, the timescale
and amplitude of variability observed at the magnitude *m*_{v} are
also listed, taken from this paper and from Guibin et al. (1995) and from
another paper submitted to AA for OJ 287. The observed values between
brackets are from Barbieri et al. (1979) for PHL 658,
Kidger et al. (1990) for
3C 345 and Moles et al. (1985) for 2128-123. The *B* mag given by
Barbieri et al. (1979) and Kidger et al. (1990) and the *I* mag for OJ 287 were normalized to
the value in *V* band (showed between brackets) using the known colour index
and assuming that the variations in *V* band are the same as those in *B* and
*I* band. The value of is calculated according to the formula and the
parameter values given by Ghisellini et al. (1993) except for the parameters
of *F*_{x} and that are replaced by the flux at the corrected *V*
magnitude during our observations of variability and by respectively.

It will be seen from Table 3 (click here) that the efficiencies derived from our observations are larger than 0.1 except for PHL 658, 3C 345 and 3C 120. However, other observations show that 0.1 for PHL 658 and 3C 345; The values for the objects are larger than one except for PKS 0754+101 and PKS 2254+074, both of which have . If , the Doppler factor should be larger than one in the relativistic beaming model. In conclusion, it seems that relativistic beaming exists for all objects.

**Table 3:** Properties of nine objects with flat spectra and known redshifts

*Acknowledgements*

The authors express their thanks to staff of the Yunnan Base, Optical Union Opening Observatory and of the computer department for their assistance during observations and data processing. We thanks the astronomical committee of the Chinese Academy of Science for financial support.