  X-ray observations of the accreting X-ray pulsar 4U~1907+09, obtained during
February 1996 with the Proportional Counter Array on the Rossi X-ray Timing
Experiment (RXTE), have enabled the first measurement of the intrinsic pulse
period Ppulse since 1984: Ppulse=440.341[+0.012,-0.017] s. 4U 1907+09 is in a
binary system with a blue supergiant. The orbital parameters were solved and
this enabled the correction for orbital delay effects of a measurement of
Ppulse obtained in 1990 with Ginga. Thus, three spin down rates could be
extracted from four pulse periods obtained in 1983, 1984, 1990, and 1996. These
are within 8% equal to a value of dPpulse/dt=+0.225 s/yr. This suggest that the
pulsar is perhaps in a monotonous spin down mode since its discovery in 1983.
Furthermore, the RXTE observations show transient ~18 s oscillations during a
flare that lasted about 1 hour. The oscillations may be interpreted as
Keplerian motion of an accretion disk near the magnetospheric radius. This, and
the notion that the co-rotation radius is much larger than any conceivable
value for the magnetospheric radius (because of the long spin period), renders
it unlikely that this pulsar spins near equilibrium like is suspected for other
slowing accreting X-ray pulsars. We suggest as an alternative that perhaps the
frequent occurrence of a retrograde transient accretion disk may be
consistently slowing the pulsar down. Further observations of flares can
provide more evidence of this.
