Period Analysis of the Semi-Detached Binary V505 Sgr

The O—C curve of the Algol—type semi—detached binary V505 Sgr has been analyzed using a new method proposed by Kalimeris et al(1994). It is shown that the orbital period oscillates with an amplitude of 0.453×10^-5days and a period of about 41.5 years that dif-fers from the previons period of 105 years. The physical mechanisms explained the orbital period behaviour have been studied. It is indicated that the present period change may not be caused by the exist of the third body proposed previously. The presence of a fourth body with mass 0.013 M_⊙ and orbit semimajor axis 17.35AU or the magnetic activity cycle of the secondary component may be used to explain the present period change.