Context : 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 10 ^ { 3 - 4 } , the source was classified as a Supergiant Fast X-ray Transient .

Aims : 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 .

Methods : 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 .

Results : INTEGRAL revealed an unusually long orbital period of 30.0 \pm 0.2 d and an elapsed accretion phase of \sim 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 \sim 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 .

Conclusions :