Discovery of a 30 day period in the supergiant fast X-ray transient SAX   J1818.6-1703
  SAX J1818.6-1703 has been characterised as a Supergiant Fast X-ray Transient system on the basis of several INTEGRAL/IBIS detections since the original BeppoSAX Wide Field Camera detection. Using IBIS/ISGRI, Swift/BAT and archival observations, we show that in fact SAX J1818.6-1703 exhibits emission on a period of 30 +/- 0.1 days, with a high degree of recurrence. SAX J1818.6-1703 is therefore the second SFXT shown to exhibit periodic outbursts, but with a considerably shorter period than the other known system, IGR J11215-5952. 
A. J. Bird, A. Bazzano, A.B. Hill, V.A. McBride, V. Sguera, S.E. Shaw,   H. J. Watkins