  Aims. We aim at detecting the presence of companions inside the inner
hole/gap region of a sample of five well known transitional disks using
spatially-resolved imaging in the near-IR with the VLT/NACO/S13 camera, which
probes projected distances from the primary of typically 0.1 to 7 arcsec. The
sample includes the stars DoAr 21, HD 135344B (SAO 206462), HR 4796A, T Cha,
and TW Hya, spanning ages of less than 1 to 10 Myr, spectral types of A0 to K7,
and hole/gap outer radii of 4 to 100 AU. Methods. In order to enhance the
contrast and to avoid saturation at the core of the point-spread function
(PSF), we use narrow-band filters at 1.75 and 2.12 {\mu}m. The "locally
optimized combination of images" (LOCI) algorithm is applied for an optimal
speckle noise removal and PSF subtraction, providing an increase of 0.5-1.5 mag
in contrast over the classic method. Results. With the proviso that we could
have missed companions owing to unfavorable projections, the VLT/NACO
observations rule out the presence of unresolved companions down to an inner
radius of about 0".1 from the primary in all five transitional disks and with a
detection limit of 2 to 5 mag in contrast. In the disk outer regions the
detection limits typically reach 8 to 9 mag in contrast and 4.7 mag for T Cha.
Hence, the NACO images resolve part of the inner hole/gap region of all disks
with the exception of TW Hya, for which the inner hole is only 4 AU. The
5{\sigma} sensitivity profiles, together with a selected evolutionary model,
allow to discard stellar companions within the inner hole/gap region of T Cha,
and down to the substellar regime for HD 135344B and HR 4796A. DoAr 21 is the
only object from the sample of five disks for which the NACO images are
sensitive enough for a detection of objects less massive than \sim 13 MJup that
is, potential giant planets or low-mass brown dwarfs at radii larger than \sim
76 AU (0".63).
