  Aims: We want to detect and quantify observables related to accretion
processes occurring locally in circumstellar disks, which could be attributed
to young forming planets. We focus on objects known to host protoplanet
candidates and/or disk structures thought to be the result of interactions with
planets. Methods: We analyzed observations of 6 young stars (age $3.5-10$ Myr)
and their surrounding environments with the SPHERE/ZIMPOL instrument on the VLT
in the H$\alpha$ filter (656 nm) and a nearby continuum filter (644.9 nm).
Results: We re-detect the known accreting M-star companion HD142527 B with the
highest published signal to noise to date in both H$\alpha$ and the continuum.
We derive new astrometry ($r = 62.8^{+2.1}_{-2.7}$ mas and $\text{PA} =
(98.7\,\pm1.8)^\circ$) and photometry ($\Delta$N_Ha=$6.3^{+0.2}_{-0.3}$ mag,
$\Delta$B_Ha=$6.7\pm0.2$ mag and $\Delta$Cnt_Ha=$7.3^{+0.3}_{-0.2}$ mag) for
the companion in agreement with previous studies, and estimate its mass
accretion rate ($\dot{M}\approx1-2\,\times10^{-10}\,M_\odot\text{ yr}^{-1}$). A
faint point-like source around HD135344 B (SAO206462) is also investigated, but
a second deeper observation is required to reveal its nature. No other
companions are detected. In the framework of our assumptions we estimate
detection limits at the locations of companion candidates around HD100546,
HD169142 and MWC758 and calculate that processes involving H$\alpha$ fluxes
larger than $\sim8\times10^{-14}-10^{-15}\,\text{erg/s/cm}^2$
($\dot{M}>10^{-10}-10^{-12}\,M_\odot\text{ yr}^{-1}$) can be excluded.
Furthermore, flux upper limits of $\sim10^{-14}-10^{-15}\,\text{erg/s/cm}^2$
($\dot{M}<10^{-11}-10^{-12}\,M_\odot \text{ yr}^{-1}$) are estimated within the
gaps identified in the disks surrounding HD135344B and TW Hya.
