  Intensity interferometry in noncentral heavy ion collisions provides access
to novel information on the geometry of the effective pion-emitting source. We
demonstrate analytically that, even for vanishing pair momentum, the cross
terms $R_{ol}^2$ and $R_{sl}^2$ of the HBT correlation function in general show
a strong first harmonic in their azimuthal dependence. The strength of this
oscillation characterizes the tilt of the major axis of the spatial emission
ellipsoid away from the direction of the beam. Event generator studies indicate
that this tilt can be large (>20 degrees) at AGS energies which makes it by far
the most significant azimuthally sensitive HBT signal at these energies.
Moreover, transport models suggest that for pions this spatial tilt is directed
opposite to the tilt of the directed flow ellipsoid in momentum space. A
measurement of the azimuthal dependence of the HBT cross terms $R_{ol}^2$ and
$R_{sl}^2$ thus probes directly the physical origin of directed pion flow.
