  The variable source OGLE-UCXB-01 is the first OGLE-discovered ultracompact
X-ray binary (UCXB). The 12-year long-term OGLE optical photometry of this
source shows a period of P= 12.8 min and a fast period decreasing rate Pdot=
-9.2E-11 s s^-1. At a luminosity of L_X ~ 4E33 erg s^-1, its X-ray emission is
also variable and correlated with the optical variability. To determine the
nature of this variable source, specifically the masses and types of its binary
components, we consider first an attractive possibility that the optical
variation is due to the secondary's ellipsoidal variation and a strong
gravitational wave emission drives the orbital decay. However, we can not find
an allowable solution to the secondary that satisfies simultaneously the three
constraints: an ultra-tight orbit, the bright absolute magnitude, and the large
amplitude of the brightness variation. Moreover, the inferred mass transfer
rate is too high. This scenario is therefore ruled out. We then find the system
is fully consistent with an "intermediate polar" model, in which the optical
and X-ray emission comes from a magnetized white dwarf (WD) accreting from a
low-mass (<~ 0.7 M_sun) main-sequence secondary. The observed period decay is
the accretion-driven spin-up of the WD. The WD spin period is 12.8 min and the
orbital period is shorter than 10 hr. The method presented here can be applied
to other UCXB candidates or impostors with time-domain data available only.
