Abstract-The diffusion of wearable and mobile devices for the acquisition and analysis of cardiac signals drastically increased the possible applicative scenarios of biometric systems based on electrocardiography (ECG). Moreover, such devices allow for comfortable and unconstrained acquisitions of ECG signals for relevant time spans of tens of hours, thus making these physiological signals particularly attractive biometric traits for continuous authentication applications. In this context, recent studies showed that the QRS complex is the most stable component of the ECG signal, but the accuracy of the authentication degrades over time, due to significant variations in the patterns for each individual. Adaptive techniques for automatic template update can therefore become enabling technologies for continuous authentication systems based on ECG characteristics.In this work, we propose an approach for unsupervised periodical re-enrollment for continuous authentication, based on ECG signals. During the enrollment phase, a "super" template obtained from a fixed number of samples is stored in the gallery. In continuous authentication, an update condition is periodically verified. If the condition is satisfied, confirming that the fresh data pertain to the stored identity, an update strategy is applied to fuse the fresh data with the "super" template. Different update conditions and update strategies are presented and evaluated.Tests have been performed on a significantly large public dataset of 24h Holter signals acquired in uncontrolled conditions, proving that the proposed approach obtains a relevant accuracy, which increases with respect to traditional biometric approaches based on a single enrolled template for each individual.