Systematic variation of the residual of PSR B 1937 + 21 , the trace of its companion star ?

PSR B1937+21 was the first millisecond pulsar ever measured. Which has been appeared as a singular pulsar. The high-precision observation of this pulsar shows systemic long-term variation in the residual of time of arrivals. This paper modelled the secular variation by the orbital precession induced time delay of a binary pulsar system. The fitting requires that as a binary pulsar, PSR B1937+21 should have small companion star, m 2 ∼ 10 −2 M ⊙ , and projected semi-major axis, x ∼ 10 −4 s to 10 −3 s. Which corresponds to ignorable radial velocity of the pulsar to the line of sight. This might explain why it has been measured as a singular pulsar instead of a binary pulsar.