Search for Intra-day Optical Variability in $\gamma$-ray--loud Blazars
  S5 0716+714 and 3C 273
  We present the photometric observations of blazars S5 0716+714 and 3C 273
with high temporal resolution (30--60s) in the $I$ or $R$ bands. The
observations were performed with a 1.02 m optical telescope from 2007 March 07
to 2012 May 16. The $F$-test, one-way analysis of variance (ANOVA) test, and
$z$-transformed discrete correlation function (ZDCF) cross-correlation zero lag
test are used to search for intra-day variability (IDV). Four and five reliable
IDVs survive three tests for S5 0716+714 and 3C 273, respectively. IDVs are
found for S5 0716+714 and 3C 273. A flare on 2008 May 08 has $\Delta I \approx$
0.06$\pm$0.01 mag in a duration of 0.54 hr for S5 0716+714. A flare on 2011 May
10 shows $\Delta R \approx$ 0.05$\pm$0.01 mag in a duration of 0.40 hr for 3C
273. Sharp dips appear on 2011 May 9 for 3C 273, and show $\Delta R \approx$
0.05$\pm$0.01 mag. Under the assumptions that the IDV is tightly connected to
black hole mass, $M_{\bullet}$, and that the flare durations are representative
of the minimum characteristic timescales, we can estimate upper bounds to
$M_{\bullet}$. In the case of the Kerr black holes, $M_{\bullet} \la 10^{8.91}
M_{\odot}$ and $M_{\bullet} \la 10^{9.02} M_{\odot}$ are given for S5 0716+714
and 3C 273, respectively. These mass measurements are consistent with those
measurements reported in the literatures. Also, we discuss the origins of
optical variations found in this work.
