  Using a data sample of 122 million Upsilon(4S) -> BBbar decays collected with
BaBar detector at the PEP-II asymmetric B factory at SLAC, we measure the
time-dependent-asymmetry parameters of the longitudinally polarized component
in the B0 -> rho^+ rho^- decay as C_L = -0.23 +/- 0.24 (stat) +/- 0.14 (syst)
and S_L = -0.19 +/- 0.33 (stat) +/- 0.11 (syst). The B0 -> rho0 rho0 decay mode
is also searched for in a data sample of about 227 million BBbar pairs. No
significant signal is observed, and an upper limit of 1.1 * 10-6 (90% C.L.) on
the branching fraction is set. The penguin contribution to the CKM angle
$\alpha$ uncertainty is measured to be 11 degree. All results are preliminary.
