We investigate the scaling properties of the spin interfaces in the Ashkin-Teller model. These interfaces are a very simple instance of lattice curves coexisting with a fluctuating degree of freedom, which renders the analytical determination of their exponents very difficult. One of our main findings is the construction of boundary conditions which ensure that the interface still satisfies the Markov property in this case. Then, using a novel technique based on the transfer matrix, we compute numerically the left-passage probability, and our results confirm that the spin interface is described by an SLE in the scaling limit. Moreover, at a particular point of the critical line, we describe a mapping of Ashkin-Teller model onto an integrable 19-vertex model, which, in turn, relates to an integrable dilute Brauer model.Another current research direction is the study of "extended" SLEs, which are curve models determined by two random processes: the driving process of the Loewner chain, and an additional process corresponding, in the associated statistical model, to local variables which are not fixed locally by the presence of the curve. It was argued [6,7] that the martingale conditions for this pair of processes correspond to null-state equations in an extended CFT. In particular, the presence of the additional process affects the relation between the central charge c and the SLE parameter κ. An important point that needs to be considered in this context, is that in the presence of additional variables, the lattice interface no longer satisfies the Markov property, which is an essential ingredient of the SLE model.The Ashkin-Teller (AT) model is a simple local spin model, with a critical line of constant central charge c = 1 and varying exponents. We believe it presents the two features described above: first, although it has been much studied with both lattice and CFT techniques, the scaling theory for its spin interfaces is not identified, and second, these interfaces leave some fluctuating variables, which makes them good candidates for extended SLEs.This paper presents two advances in the study of spin interfaces, on the particular example of the AT model. First, we introduce some specific boundary conditions (BCs) which ensure that the spin interface satisfies the Markov property, even in the presence of an additional variable. We then verify numerically that our interface relates to SLE, and for this purpose, we introduce a new algorithm allowing the measurement of the left-passage probability [8] in the transfer-matrix formalism. Our results also include an accurate numerical determination of the fractal dimension d f of spin interfaces, also by means of the transfer matrix, which supports a conjecture for d f stated in [9]. Second, we identify a specific value on the critical line of the AT model, where the temperature tends to zero, and at this point, we describe an exact mapping of the AT model onto an integrable 19-vertex model. Although this second result does not give direct access to spin interface ...