1998NewA....3..435H

We have combined Rossi X-ray Timing Explorer observations of X2127+119, the low-mass X-ray binary in the globular cluster M15, with archival X-ray lightcurves to study the stability of the 17.1 hr orbital period. We find that the data cannot be fit by the Ilovaisky et al. (1993) ephemeris, and requires either a 7sigma change to the period or a period derivative P/P ∼ 9 x 10^ - 7 yr^ - 1. Given its remarkably low L_X/Lopt such a P lends support to models that require super-Eddington mass transfer in a q ∼ 1 binary.