  Blue large-amplitude pulsators (BLAPs) make up a rare class of hot pulsating
stars with effective temperatures of $\approx$30,000 K and surface gravities of
4.0 - 5.0 dex (cgs). The evolutionary origin and current status of BLAPs is not
well understood, largely based on a lack of spectroscopic observations and no
available mass constraints. However, several theoretical models have been
proposed that reproduce their observed properties, including studies that
identify them as pulsating helium-core pre-white dwarfs (He-core pre-WDs). We
present here follow-up high-speed photometry and phase-resolved spectroscopy of
one of the original 14 BLAPs, OGLE-BLAP-009, discovered during the Optical
Gravitational Lensing Experiment. We aim to explore its pulsation
characteristics and determine stellar properties such as mass and radius in
order to test the consistency of these results with He-core pre-WD models.
Using the mean atmospheric parameters found using spectroscopy, we fit a
spectral energy distribution to obtain a preliminary estimate of the radius,
luminosity and mass by making use of the Gaia parallax. We then compare the
consistency of these results to He-core pre-WD models generated using MESA,
with predicted pulsation periods implemented using GYRE. We find that our mass
constraints are in agreement with a low-mass He-core pre-WD of $\approx$0.30
M$_{\odot}$.
