  Using BATSE and RXTE observations from 1991 April to 2001 August we have
detected 71 outbursts from 82 periastron passages of EXO 2030+375, a 42-second
transient X-ray pulsar with a Be star companion, including several outbursts
from 1993 August to 1996 April when the source was previously believed to be
quiescent. Combining BATSE, RXTE, and EXOSAT data we have derived an improved
orbital solution. Applying this solution results in a smooth profile for the
spin-up rate during the giant outburst and results in evidence for a
correlation between the spin-up rate and observed flux in the brighter BATSE
outbursts. Infrared and Halpha measurements show a decline in the density of
the circumstellar disk around the Be star. This decline is followed by a sudden
drop in the X-ray flux and a turn-over from a spin-up trend to spin-down in the
frequency history. This is the first Be/X-ray binary which shows an extended
interval, about 2.5 years, where the global trend is spin-down, but the
outbursts continue. In 1995 the orbital phase of EXO 2030+375's outbursts
shifted from peaking about 6 days after periastron to peaking before
periastron. The outburst phase slowly recovered to peaking at about 2.5 days
after periastron. We interpret this shift in orbital phase followed by a slow
recovery as evidence for a global one-armed oscillation propagating in the Be
disk. This is further supported by changes in the shape of the Halpha profile
which are commonly believed to be produced by a reconfiguration of the Be disk.
The truncated viscous decretion disk model provides an explanation for the long
series of normal outbursts and the evidence for an accretion disk in the
brighter normal outbursts.
