  We present the most recent results from our investigation on Supergiant Fast
X-ray Transients, a class of High-Mass X-ray Binaries, with a possible
counterpart in the gamma-ray energy band. Since 2007 Swift has contributed to
this new field by detecting outbursts from these fast transients with the BAT
and by following them for days with the XRT. Thus, we demonstrated that while
the brightest phase of the outburst only lasts a few hours, further activity is
observed at lower fluxes for a remarkably longer time, up to weeks.
Furthermore, we have performed several campaigns of intense monitoring with the
XRT, assessing the fraction of the time these sources spend in each phase, and
their duty cycle of inactivity.
