Photometric study of the eclipsing blue straggler V205 in the globular cluster NGC 5139

B and V light curves of an EA-type binary V205 in the globular cluster NGC 5139 are analyzed by the W-D program. We found that V205 is possibly a detached binary and the mass ratio is 0.1596. The secondary component is touching or nearly touching its inner Roche Lobe. By studying the O−C diagram of V205, we discovered that the orbital period is continuously decrease at a rate of dp/dt=−1.89(±0.01)×10−7 d yr−1 and should be caused by angular momentum and mass loss. The angular momentum loss will drive it evolve into a contact binary. Since V205 is a proper motion member of NGC 5139, we estimated its absolute parameters based on the distance modulus of the cluster and determined that: a=2.50R⊙,M1=0.76M⊙,R1=1.14R⊙,L1=5.46L⊙,M2=0.12M⊙,R2=0.52R⊙, and L2=0.70L⊙. V205 occupied the blue straggler stars on the color-magnitude diagram of NGC 5139. It is an eclipsing blue straggler and is most possibly formed by mass transfer between the two components. Since original short-period systems similar to V205 should be evolved in such a long life time of the globular cluster, the short-period binary should undergo special evolutionary stages. High accuracy photometric and high resolution spectral observations are essential for this unusual system.