Context. The multiplicity rate of the youngest stellar objects is directly linked to their formation process, and therefore represents one of the most critical parameters to constrain theories of star formation. Multiplicities of embedded protostars are, however, not well determined empirically. Aims. The aim of this work is to study the multiplicity of three embedded proto-stellar systems, namely SSV 63, L1551 NE, and L1551-IRS5, through mid-IR imaging. All of them are suspected binary or

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... ple systems, but prone to ambiguities in the literature. Methods. We present high spatial resolution mid-infrared observations obtained with VISIR at the VLT. Results. For the SSV 63 system, we report the discovery of a new Class I companion, approx 2. 6 N of SSV63 E, which was never seen in previous near-IR and radio continuum studies. The companion to SSV 63 W, which was detected at near-IR wavelengths, is also present in the mid-IR regime. In the case of L1551 NE, we have confirmed the presence of a companion at 0. 5 NW from the central star, as previously reported by Reipurth et al. (2002 , AJ, 124, 1045. Finally, we have not detected any companion to L1551-IRS5. However, we find signs of extended emission around the target. In total, we have detected three binaries. The SEDs of the primaries are consistent with their Class I/flat-spectrum classification. For the secondaries, ancillary data are required to derive their evolutionary status. Conclusions. The geometrical arrangement of sources in the SSV63 system strongly supports the view of hierarchical fragmentation of cloud cores.