Law of variation of the SS 433 precession period, 1978--1981

From spectroscopic observations of SS 433 with the 6-m telescope an effort is made to fit an analytic law to the shortening of the precession period P during 1978--1981. For 85% of the time that SS 433 has been monitored, the P variation can be represented by the function P = P0 exp]-[(t-t0)/tau]/sup α/], with an index α = 3. At the initial epoch t0 = JD 2 433 448 +- 10 the period P0 = 169/sup d/.55 +- 0/sup d/.10, and the relaxation time tau = 10.33 +- 0.10 yr. Between November 1980 and October 1981 there was a very smooth transition to a similar law with α = 4 and tau = 7.9 yr. During 39 months of observation the rate of period shortening has changed from P = -0.0005 +- 0.0002 to P = -0.0197 +- 0.0008