Among the competing evolution theories for subdwarf-B ( sdB ) stars is the binary evolution scenario . EC 20117-4014 ( =V4640 Sgr ) is a spectroscopic binary system consisting of a pulsating sdB star and a late F main-sequence companion \citep O ’ Donoghue et al.1997 , however the period and the orbit semi-major axes have not been precisely determined . This paper presents orbital characteristics of the EC 20117-4014 binary system using 20 years of photometric data . Periodic Observed minus Calculated ( O-C ) variations were detected in the two highest amplitude pulsations identified in the EC 20117-4014 power spectrum , indicating the binary system ’ s precise orbital period ( P = 792.3 days ) and the light-travel time amplitude ( A = 468.9 s ) . This binary shows no significant orbital eccentricity and the upper limit of the eccentricity is 0.025 ( using 3 \sigma as an upper limit ) . This upper limit of the eccentricity is the lowest among all wide sdB binaries with known orbital parameters . This analysis indicated that the sdB is likely to have lost its hydrogen envelope through stable Roche lobe overflow , thus supporting hypotheses for the origin of sdB stars . In addition to those results , the underlying pulsation period change obtained from the photometric data was \dot { P } = 5.4 ( \pm 0.7 ) \times 10 ^ { -14 } d d ^ { -1 } , which shows that the sdB is just before the end of the core helium-burning phase .