Context. (Pre-)Transitional disks show gaps and cavities that can be related to ongoing planet formation. According to theory, young embedded planets can accrete material from the circumplanetary and circumstellar disks and can be detected using accretion tracers, such as the H α emission line. Aims. We aim to detect accreting protoplanets within the cavities of five (pre-)transitional disks through adaptive-optics(AO)-assisted spectral angular differential imaging in the optical regime. Methods. We performed simu…