  We report the detection of 8.914-hr variability in both optical and
ultraviolet light curves of LP 40-365 (also known as GD 492), the prototype for
a class of partly burnt runaway stars that have been ejected from a binary due
to a thermonuclear supernova event. We first detected this 1.0% amplitude
variation in optical photometry collected by the Transiting Exoplanet Survey
Satellite. Re-analysis of observations from the Hubble Space Telescope at the
TESS period and ephemeris reveal a 5.8% variation in the ultraviolet of this
9800 K stellar remnant. We propose that this 8.914-hr photometric variation
reveals the current surface rotation rate of LP 40-365, and is caused by some
kind of surface inhomogeneity rotating in and out of view, though a lack of
observed Zeeman splitting puts an upper limit on the magnetic field of <20 kG.
We explore ways in which the present rotation period can constrain progenitor
scenarios if angular momentum was mostly conserved, which suggests that the
survivor LP 40-365 was not the donor star but was most likely the bound remnant
of a mostly disrupted white dwarf that underwent advanced burning from an
underluminous (Type Iax) supernova.
