  We report a precision measurement of a tune-out wavelength for 87Rb using
circularly polarized light. A tune-out wavelength characterizes a zero in the
electric polarizability of the atom. For circularly polarized light, the total
polarizability depends on both the scalar and vector polarizability components.
This shifts the location of the tune-out wavelength and makes it sensitive to
different combinations of atomic dipole matrix elements than the scalar
polarizability alone. Using $\sigma_-$ polarized light with an an estimated
purity of 0.9931(1), we observe a tune-out wavelength of 785.1522(3) nm, which
agrees with theoretical expectations when small contributions from the core
electrons and off-resonant valence states are taken into account.
