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Published online by Cambridge University Press: 12 April 2016
In this paper we present evidence that several of the defining parameters of the Kinematic Model for SS433 are not constant but rather exhibit long term systematic changes. Recent data confirm the existence of the previously reported decrease in the precessional period. The value for this period change, when combined with the observed change in the period of the synodic spectral variations, implies that the orbital period is not significantly changing on a time scale less than 1000 years.
In addition we find mounting evidence for a statistically significant (40) secular change in the cone angle θ at a rate of about -1.5 x 10-3 deg/day. However, t the surprisingly short time scales implied by the observed values of P and θ when combined with estimates of the system age suggest the possible existence of detectable higher time derivatives. This view is supported by the most recent data which suggest a value for P ~ 10-5 (days)-1. It is possible to understand these secular changes in terms of the motions to be expected from an object exhibiting classical precession in response to an external torque.