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GJ758 B is a brown dwarf companion to a nearby (15.76 pc) solar-type,metal-rich (M/H = +0.2 dex) main-sequence star (G9V) that was discovered withSubaru/HiCIAO in 2009. From previous studies, it has drawn attention as beingthe coldest (~600K) companion ever directly imaged around a neighboring star.We present new high-contrast data obtained during the commissioning of theSPHERE instrument at the VLT. The data was obtained in Y-, J-, H-, and Ks-bandswith the dual-band imaging (DBI) mode of IRDIS, providing a broad coverage ofthe full near-infrared (near-IR) range at higher contrast and better spectralsampling than previously reported. In this new set of high-quality data, wereport the re-detection of the companion, as well as the first detection of anew candidate closer-in to the star. We use the new 8 photometric points for anextended comparison of GJ758 B with empirical objects and 4 families ofatmospheric models. From comparison to empirical object, we estimate a T8spectral type, but none of the comparison object can accurately represent theobserved near-IR fluxes of GJ758 B. From comparison to atmospheric models, weattribute a Teff = 600K $\pm$ 100K, but we find that no atmospheric model canadequately fit all the fluxes of GJ758 B. The photometry of the new candidatecompanion is broadly consistent with L-type objects, but a second epoch withimproved photometry is necessary to clarify its status. The new astrometry ofGJ758 B shows a significant proper motion since the last epoch. We use thisresult to improve the determination of the orbital characteristics using twofitting approaches, Least-Square Monte Carlo and Markov Chain Monte Carlo.Finally, we analyze the sensitivity of our data to additional closer-incompanions and reject the possibility of other massive brown dwarf companionsdown to 4-5 AU. [abridged]

First light of the VLT planet finder SPHERE. I. Detection and characterization of the sub-stellar companion GJ 758 B