  The PHENIX Collaboration has reported third-order harmonic oscillations of
the source radius parameters when measuring the Hanbury Brown-Twiss correlation
function for charged hadrons relative to the triangular flow angle. We explore
possible origins of such third-order oscillations with a simple Gaussian source
featuring both a triangular geometric deformation and triangular flow.
Third-order oscillations of the HBT radii can arise from a purely geometric
triangular deformation superimposed on an azimuthally symmetric radial flow, or
from a radially symmetry spatial distribution which expands anisotropically
with a triangular component in the flow velocity profile. In both cases the
final particle momentum distribution features triangular flow. We show that the
two alternatives can be distinguished experimentally through the phase of the
azimuthal oscillations of the HBT radii relative to the triangular flow plane.
