  We present optical light curves of 19 radio quiet (RQ) broad absorption line
(BAL) QSOs and study their rapid variability characteristics. Systematic CCD
observations, aided by a careful data analysis procedure, have allowed us to
clearly detect any such microvariability exceeding 0.01--0.02 mag. Our
observations cover a total of 13 nights (~72 hours) with each quasar monitored
for about 4 hours on a given night. Our sample size is a factor of three larger
than the number of radio-quiet BALQSOs previously searched for
microvariability. We introduce a scaled F-test statistic for evaluating the
presence of optical microvariability and demonstrate why it is generally
preferable to the statistics usually employed for this purpose. Considering
only unambiguous detections of microvariability we find that ~11 per cent of
radio-quiet BALQSOs (two out of 19 sources) show microvariability for an
individual observation length of about 4 hr. This new duty cycle of 11 per cent
is similar to the usual low microvariability fraction of normal RQQSOs with
observation lengths similar to those of ours. This result provides support for
models where radio-quiet BALQSO do not appear to be a special case of the
RQQSOs in terms of their microvariability properties.
