The O–C curve of the Algol-type eclipsing binary SW Cyg was analyzed using the Kalimeris method. The observed period variations with time, the P(E) function and its rate of change dP/dE, were calculated. The plots of O–C values as well as of P(E) and dP/dE against ephemeris (E) all show rather regular period variations in this system. To reveal any cyclic period variations, the P(E) function was subjected to Fourier analysis. A cyclic period change of average ≈27.8-yr duration was obtained. Also a relative mass transfer rate of Δm/m = −1.1 × 10−9 yr−1 was estimated. Finally the existence of a third companion is suggested, and possible causes of period variations in the system are discussed.
