2003A&A...399..231C

We have analysed the pulse arrival time data of the pulsar B1557-50 collected over a period of nearly 13 years, from 1986 July to 1999 May, in order to investigate the long-term spin behaviour. The results reveal sustained cyclic variations in both the timing residuals and dispersion measure (DM) of this object. Both the timing and DM sequences are well described by two harmonically-related periodicities of ∼1600 and 800 days. The variations in spin-down rate and DM are significantly anticorrelated in the sense that maximum {dot}(ν) corresponds to minimum DM. Interpretation of the data in terms of free precession of an isolated radio pulsar suggests a wobble angle θ≃0.01°. The data are consistent with at least 6.5% of the stellar moment of inertia being coupled to the crust.