  Spin transfer from circularly polarized real photons to recoiling hyperons
has been measured for the reactions $\vec\gamma + p \to K^+ + \vec\Lambda$ and
$\vec\gamma + p \to K^+ + \vec\Sigma^0$. The data were obtained using the CLAS
detector at Jefferson Lab for center-of-mass energies $W$ between 1.6 and 2.53
GeV, and for $-0.85<\cos\theta_{K^+}^{c.m.}< +0.95$. For the $\Lambda$, the
polarization transfer coefficient along the photon momentum axis, $C_z$, was
found to be near unity for a wide range of energy and kaon production angles.
The associated transverse polarization coefficient, $C_x$, is smaller than
$C_z$ by a roughly constant difference of unity. Most significantly, the {\it
total} $\Lambda$ polarization vector, including the induced polarization $P$,
has magnitude consistent with unity at all measured energies and production
angles when the beam is fully polarized. For the $\Sigma^0$ this simple
phenomenology does not hold. All existing hadrodynamic models are in poor
agreement with these results.
