  This paper is concerned with the optimized Schwarz waveform relaxation method
and Ventcel transmission conditions for the linear advection-diffusion
equation. A mixed formulation is considered in which the flux variable
represents both diffusive and advective flux, and Lagrange multipliers are
introduced on the interfaces between nonoverlapping subdomains to handle
tangential derivatives in the Ventcel conditions. A space-time interface
problem is formulated and is solved iteratively. Each iteration involves the
solution of time-dependent problems with Ventcel boundary conditions in the
subdomains. The subdomain problems are discretized in space by a mixed hybrid
finite element method based on the lowest-order Raviart-Thomas space and in
time by the backward Euler method. The proposed algorithm is fully implicit and
enables different time steps in the subdomains. Numerical results with
discontinuous coefficients and various Pecl\'et numbers validate the accuracy
of the method with nonconforming time grids and confirm the improved
convergence properties of Ventcel conditions over Robin conditions.
