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Transition disks have large central cavities that have been spatiallyresolved during recent years. Cavities and other substructures in circumstellardisks are often interpreted as signposts to massive companions. We aim tosearch for stellar and substellar companions in the central regions oftransition disks. We want to determine if these disks might be circumbinary intheir nature, similar to the HD 142527 system. We observed four systems, HD100453, HD 100546, HD 135344 B, and PDS 70, with the sparse aperture maskingmode of VLT/SPHERE. We extracted the complex visibilities and bispectra fromthe H2 and H3 imaging data. A binary model was fit to the closure phases tosearch for companions and estimate detection limits. For validation, we alsoanalyzed four archival datasets of HD 142527 and inferred the orbital elementsand atmospheric parameters of its low-mass stellar companion. We have notdetected any significant point sources in the four observed systems. With acontrast sensitivity of $\approx$0.004, we can rule out stellar companions downto $\approx$2 au and partially explore the substellar regime at separations$\gtrsim$3-5 au. The analysis of HD 142527 B revealed that its projected orbitis aligned with dust features in the extended inner disk and that the orbitcould be close to coplanar with the outer disk. Atmospheric modeling confirmsthe low-gravity and slightly reddened spectral appearance. The bulk parametersare in agreement with dynamical constraints and evolutionary tracks. Incontrast to HD 142527, we find no evidence that a close-in stellar companion isresponsible for the resolved disk features of HD 100453, HD 100546, HD 135344B, and PDS 70. Instead, the formation of giant planets or even low-mass browndwarfs could be shaping the innermost environment ($\lesssim$20 au) of thesecircumstellar disks, as is the case with the planetary system of PDS 70.

Searching for low-mass companions at small separations in transition disks with aperture masking interferometry