  SAX J1818.6-1703 is a flaring transient X-ray source serendipitously
discovered by BeppoSAX in 1998 during an observation of the Galactic centre.
The source was identified as a High-Mass X-ray Binary with an OB SuperGiant
companion. Displaying short and bright flares and an unusually very-low
quiescent level implying intensity dynamical range as large as 1e3-4, the
source was classified as a Supergiant Fast X-ray Transient. The mechanism
triggering the different temporal behaviour observed between the classical
SGXBs and the recently discovered class of SFXTs is still debated. The
discovery of long orbits (>15 d) should help to discriminate between emission
models and bring constraints.
  We analysed archival INTEGRAL data on SAX J1818.6-1703. We built short- and
long-term light curves and performed timing analysis in order to study the
temporal behaviour of SAX J1818.6-1703 on different time scales. INTEGRAL
revealed an unusually long orbital period of 30.0+/-0.2 d and an elapsed
accretion phase of ~6 d in the transient SGXB SAX J1818.6-1703. This implies an
elliptical orbit and constraints the possible supergiant spectral type between
B0.5-1I with eccentricities e~0.3-0.4 (for average fundamental parameters of
supergiant stars). During the accretion phase, the source behaved like
classical SGXBs. The huge variations of the observed X-ray flux can be
explained through accretion of macro-clumps formed within the stellar wind. Our
analysis strengthens the model which predicts that SFXTs behave as SGXBs but
with different orbital parameters, thus different temporal behaviour.
