  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 re-analyzing RXTE ASM data, 88 data points based on observations by
MAXI, and two 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 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 15.8 yr. They can be related either to
apsidal motion in the close binary with eccentricity e=0.03 or to a presence of
a third body with the mass of about 0.7 solar masses located at a distance
about 16 au from the close binary. We also detect irregular and abrupt changes
in the residuals between the best fit ephemeris and the data. While we discuss
possible reasons for the changes, their origin remains unclear. A tentative
period of about 188 days in X-ray flux variations was found. Such a period
could be attributed to a small precessing disk around the compact object.
