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(Pre-)transitional disks show gaps and cavities that can be related withon-going planet formation. According to theory, young embedded planets canaccrete material from the circumplanetary and circumstellar disks, so that theycould be detected in accretion tracers, like the H$_{\alpha}$ emission line. Inthis work, we present spectral angular differential imaging AO-assistedobservations of five (pre-)transitional disks obtained with SPHERE/ZIMPOL atthe Very Large Telescope (VLT). They were obtained in the H$_{\alpha}$ line andthe adjacent continuum. We have combined spectral and angular differentialimaging techniques to increase the contrast in the innermost regions close tothe star, and search for the signature of young accreting protoplanets. As aresult, the reduced images do not show any clear H$_{\alpha}$ point sourcearound any of the targets. We report faint H$_{\alpha}$ emissions around TW Hyaand HD163296: while the former is most probably an artifact related with aspike, the nature of the latter remains unclear. The spectral and angulardifferential images yield contrasts of 6--8 magnitudes at separations of $\sim$100 mas from the central stars, except in the case of LkCa15, with values of$\sim$3 mag. We have estimated upper limits to the accretion luminosity ofpotential protoplanets, obtaining that planetary models provide an averagevalue of $L_{\rm acc} \sim 10^{-4}$ $L_{\odot}$ at 200 mas, which is $\sim$2orders of magnitude higher than the $L_{\rm acc}$ estimated from theextrapolation of the $L_{H_{\alpha}}$ - $L_{acc}$ stellar relationship. Weexplain the lack of protoplanet detections as a combination of differentfactors, like e.g. episodic accretion, extinction from the circumstellar andcircumplanetray disks, and/or a majority of low-mass, low-accreting planets.

Searching for H$_{\alpha}$-emitting sources in the gaps of five transitional disks. SPHERE/ZIMPOL high-contrast imaging