Measurements of charmless B decays related to alpha at BaBar

We report recent measurements of the CKM angle alpha using data collected by the BABAR detector at the PEP-II asymmetric-energy e+e- collider at the SLAC National Accelerator Laboratory. In addition to improved constraints on alpha from the decays B+ -> rho+ rho0, we also present preliminary results of neutral and charged B meson decays to K_1(1270) pi and K_1(1400) pi and its impact on the estimate for the CKM angle alpha based on time-dependent analysis of CP-violating asymmetries in B0 -> a_1(1260) pi. Moreover we report the first observation of the decay B -> a_1(1260)+ a_1(1260)-; this mode can be used, in principle, to provide an indipendent measurement of alpha.