A companion candidate in the gap of the T Chamaeleontis transitional disk

Context. T Cha is a young star surrounded by a cold disk. The presence of a gap within its disk, inferred from fitting to the spectral energy distribution, has suggested on-going planetary formation. Aims. The aim of this work is to look for very low-mass companions within the disk gap of T Cha. Methods. We observed T Cha in L' and K-s with NAOS-CONICA, the adaptive optics system at the VLT, using sparse aperture masking. Results. We detected a source in the L' data at a separation of 62 +/- 7mas, position angle of similar to 78 +/- 1 degrees, and a contrast of Delta L' = 5.1 +/- 0.2 mag. The object is not detected in the K-s band data, which show a 3-sigma contrast limit of 5.2 mag at the position of the detected L' source. For a distance of 108 pc, the detected companion candidate is located at 6.7 AU from the primary, well within the disk gap. If T Cha and the companion candidate are bound, the comparison of the L' and K-s photometry with evolutionary tracks shows that the photometry is inconsistent with any unextincted photosphere at the age and distance of T Cha. The detected object shows a very red K-s - L' color, for which a possible explanation would be a significant amount of dust around it. This would imply that the companion candidate is young, which would strengthen the case for a physical companion, and moreover that the object would be in the substellar regime, according to the K-s upper limit. Another exciting possibility would be that this companion is a recently formed planet within the disk. Additional observations are mandatory to confirm that the object is bound and to properly characterize it.