5 Conclusion

We have investigated the spin behaviour of the pulsar B1557-50 over a period of 13 years. The result shows sustained and very nearly periodic variations in both rotational and DM parameters of this object. A particularly good description of the data is given by a combination of two sinusoids with two harmonically-related frequencies of 0.00063 and 0.0013 d^-1. The observations presented here are most plausibly explained in terms of free precession of the pulsar B1557-50, requiring a wobble angle $\theta \sim 0.01^{\circ}$ and about 6.5% of the total stellar moment of inertia to be strongly coupled to the crust, and hence participates in the precession.

Acknowledgements

We thank Claire Flanagan and Johnson Urama for their helpful comments and discussions, and Mike Gaylard for critically reading through the manuscript. AEC acknowledges the support and hospitality of HartRAO Director, G.D. Nicolson. He is grateful to IAU Commission 38 for supporting his visit to HartRAO with a travel grant and to an anonymous referee for his useful comments which helped to improve this paper.