By using available archival X-ray data, we significantly extended the list of times of X-ray minima. The new list includes 65 data points obtained by critically reanalyzing RXTE ASM data, 88 data points based on observations by MAXI, and 2 data points based on observations by SUZAKU and AstroSat. Analyzing the data along with times of X-ray minima available from the literature, we provide the most accurate estimate of the rate of period change to date. We do not confirm the existence of a second derivative of the orbital period suggested by some authors earlier. Instead, we find that the changes in the period can be fit by a sum of quadratic and sinusoidal functions. The period of sinusoidal variations is 15.79yr. They can be related either to apsidal motion in the close binary with eccentricity e~0.03 or to the presence of a third body with a mass of about 0.7M_{sun}_ located at a distance ~16 au from the close binary.