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The ultra-compact Low Mass X-ray Binary (LMXB) X1916-053, composed of aneutron star and a semi-degenerated white dwarf, exhibits periodic X-ray dipswith variable width and depth. We have developed new methods to parameterizethe dip to systematically study its variations. This helps to furtherunderstand binary and accretion disk behaviors. The RXTE 1998 observationsclearly show a 4.87d periodic variation of the dip width. This is probably dueto the nodal precession of the accretion disk, although there are nosignificant sidebands in the spectrum from the epoch folding search. From thenegative superhump model (Larwood et. al. 1996), the mass ratio can beestimated as q = 0.045. Combined with more than 24 years of historical data, wefound an orbital period derivative of $\dot{P}_{orb}/P_{orb}=(1.62 \pm0.48)\times 10^{-7} yr^{-1}$ and established a quadratic ephemeris for theX-ray dips. The period derivative seems inconsistent with the prediction of thestandard model of binary orbital evolution proposed by Rappaport et. al.(1987). On the other hand, the radiation-driven model (Tavani et. al. 1991) mayproperly interpret the period derivative even though the large mass outflowpredicted by this model has never been observed in this system. With the bestephemeris, we obtained that the standard deviation of primary dips are smallerthan that of secondary dips. This means that the primary dips are more stablethan the secondary dips. Thus, we conclude that the primary dips of X1916-053occur from the bulge at the rim instead of the ring of the disk proposed byFrank et. al. (1987).

Parameterization studies of the properties of the X-ray dips for Low Mass X-ray binary X1916-053